Guide:Old One's Army strategies

The Old One's Army invasion event differs from other invasion events such as the Goblin Army in that the invading army appears where you summon it, and cannot appear on its own without a player starting the event. This means that as opposed to other invasion events, this is one that is meant to be fought in a prepared arena.

The invasion is separated into three tiers depending on player progression. Tier 3 requires the Golem to have been defeated and is the hardest version of this event. Tier 2 doesn't require Golem to be defeated, but the world should still be in hardmode with at least one mechanical boss defeated. Finally, Tier 1 is active when none of the previously listed goals have been achieved.

Player must prepare an arena in roughly flat terrain: a maximum of three tiles of vertical deviation is allowed, any more and the Eternia Crystal Stand will not accept the Eternia Crystal. Ten Etherian Mana are spawned, allowing to summon one Tavernkeep sentry immediately. Other sentries can be used as well, as during the event the sentries summoned with Etherian mana will not count against the sentry cap, however, the "normal" sentries will have to be renewed from time to time as they will not persist for the entire duration of the event.

Two mysterious portals will appear, and monsters from Old One's Army will spawn there. Players should kill them, grab the etherian mana they drop, and use it to summon additional sentries so as to tackle increasingly tougher waves.

Pre-Mechanical Bosses
Melee

Invasions like Old One's Army are hard to deal with especially with the additional objective of protecting the crystal. A focus on offense rather than defense pays off. Summoning equipment, such as the Bee armor paired with Imp minions and a Bewitching Table, can make short work of the initial waves.

Weapons with high knockback and fast speed with piercing capabilities can be helpful in the early waves. The Dark Lance with a Ruthless Modifier paired with Molten armor is good for stopping the ground troops. It's possible to place a backup Summoning set of armor in the social slots, to allow for quick switching.

Ranged

A Water Bolt, Diamond Staff or Bee's Knees with Wooden Arrows are fantastic weapons for this event. Place a vertical row of blocks behind each portal to deflect Water Bolt back the way it came, right through each enemy in its way. Pairing Celestial Cuffs with a Water Bolt allow you to beat Tier 1 easily. Just stand in 20-25 squares from any portal and constantly fire water bolts in a perfect line. They will bounce from the wall behind the portal, kill enemies on the other side and provide an unbreachable defense for Crystal. If you stand far enough away, no one will be able to hit you and all enemies will die in seconds, including the final Dark Mage.

Explosive trap abuse

Place all explosive traps at the left portal, so at the wave 5 every enemy except the Dark Mage will die. On the 5th wave kill enemies from the right portal for some time, until the Dark Mage comes out from the left portal and then focus him ignoring enemies from right portal (all enemies from left portal including wyverns will die instantly because of explosive traps). You should be able to kill him before enemies from right portal destroy the Eternia Crystal.

Tier 2
Arena: Campfire, Heart Lantern, and a platform about 20 blocks above the Eternia Crystal with a heart statue hooked up to a timer. An Ammo Box is also useful, and you should add a roof on top of the Eternia Crystal because kobold gliders die on impact to any solid block when flying. This way, your Crystal will be defended from them.

Equipment: Frost armor, Godly (or better) Dart Pistol/Rifle with Crystal Darts, Daedalus Stormbow with Holy Arrows, Demonic Lightning Aura Cane, Golden Shower (if available), Magic Quiver and Cobalt Shield variant.

Strategy: Clear the early waves with the dart pistol/rifle while setting up the lightning auras. Place one at each portal and work your way towards the Eternia Crystal. Place two auras on the platform in the air where flying enemies will enter it (if standing at the crystal, they will be about the edge of your screen).

Waves five and six contain a large amount of Etherian Wyverns and Kobold Gliders, so switch to the Stormbow in order to deal with these effectively. For the final wave, stay near the center while focusing on the Ogres and flying enemies with the Stormbow (using Golden Shower on the Ogres if you have it). If you were able to keep most of the flying enemies away from the crystal during the previous waves, you should be able to just finish the final wave.

If you are still having trouble, try warding prefixes on your accessories. Star Veil, Charm of Myths, Frozen Turtle Shell, and Flesh Knuckles are all useful accessories if you are taking too much damage, though if you hang around the center platform in later waves you will only get hit by flying enemies.

Also consider Regeneration, Ironskin, Endurance, Archery, and Ammo Reservation potions, though they are not necessary.

(May not work if done incorrectly). Due to the fact that Lihzard traps are mineable without a Picksaw, you may consider, after Plantera, entering the Lizhard Temple and stealing all the traps. Then, wire them up to 1 second timers and put them at the portal spawns. However, when you reach Tier 3, consider dismantling them and using them for something else, due to massively increased defence on the enemies.

Tier 3
Defending the Eternia Crystal is very difficult for people that just defeated the Golem. If possible, play multiplayer with your friends in a world that the Golem is defeated on. If there are only 2 people in a world, you should defend the crystal any side (right or left), and the second player should defend the other side. With this strategy, it is really easy to get to Betsy.

Expert Mode difficulty (single player):

Melee

Arena: Campfire, Heart Lantern, and two rows of platforms about 20 blocks above the Eternia Crystal with a heart statue hooked up to a timer. A Sharpening Station is also useful.

Equipment: Shinobi Infiltrator Armor, Demonic Lightning Aura Staff (Successfully farming tier 2 thirteen times will give you the 325 Defender Medals you need to get the full set of armor and staff once golem is defeated), If Shinobi Infiltrator Armor is not available, beetle armor also works, Ruthless Staff of the Frost Hydra, Godly/Ruthless Possessed Hatchet, Godly Paladin's Hammer, Vampire Knives (if available), Star Veil, Charm of Myths, Ankh Shield, Celestial Shell, Celestial Stone, Flesh Knuckles (if available)/Frozen Turtle Shell, and Blessed Apple. Warding prefix is probably best. Useful potions include Regeneration, Ironskin, Endurance, Heartreach, and Rage/Wrath potions. Use a Flask of Ichor if available.

Strategy: Set up the lightning auras right on top of the portals and as close as possible to each other leading towards the Eternia Crystal, with two frost hydras about halfway between the portal and Eternia Crystal on each side. At some point you may want two lightning auras on the platform, one on each side of the screen, to take care of any enemies that fly above the frost hydras.

Use the Paladin's hammer to take care of the mobs and Ogres/Dark Mages, especially watch out for groups of Drakin, as they may push past the frost hydras in the early stages.

Use the potions right before the final wave, make sure the frost hydras are re-summoned, and switch to the possessed hatchet. Ignore all mobs except Etherian Wyverns spawned by Betsy. Focus on dodging and allow the hatchet to whittle Betsy down. As long as the wyverns are taken care of quickly, you should have all the time you need to finish the fight, though it will take awhile.

This strategy allows you to defeat tier 3 (and Betsy) immediately after fighting Golem, though you will need to fight him multiple times in order to get the hatchet and two Sun Stones.

Ranged

Arena: Campfire, Heart Lantern, and two rows of platforms about 20 blocks above the Eternia Crystal with a heart statue hooked up to a timer. An Ammo Box is also useful.

Equipment: Red Riding armor, Demonic Explosive Trap Staff, Ruthless Staff of the Frost Hydra, Unreal Tsunami, Stake Launcher (not necessary), Golden Shower (if available), Magic Quiver, Star Veil, Frozen Turtle Shell, Cobalt Shield variant, Celestial Shell and Charm of Myths as well as a Blessed Apple. Useful potions include Regeneration, Ironskin, Endurance, Archery and Heartreach potions.

Strategy: Set up the explosive traps right on top of the portals and immediately next to each other leading towards the Eternia Crystal, with two frost hydras about halfway between the portal and Eternia Crystal on each side. Ogres will be the only thing to take care of until the final wave, but a stake launcher is useful for clearing early waves when there aren't a lot of explosive traps since it will pierce up to ten enemies.

Use the potions right before the final wave, make sure the frost hydras are re-summoned, and switch to Holy, Venom, or Ichor arrows. Holy Arrows will rain stars on any mobs that get past the explosive traps and frost hydras (there shouldn't be any), while Venom Arrows will deal the most damage (pre-Moon Lord). If you have a crimson world, Ichor Arrows or a Golden Shower are both useful against Betsy. Ignore all mobs except Etherian Wyverns spawned by Betsy in order to focus her down. Dodging and managing health is key to winning this fight, but it should be fairly straightforward.

Magic

Arena: Flat area with Conveyor Belts on both sides of Eternia Crystal bringing Stars towards the center. Additional Buffs can help but are not necessary

Equipment: Nebula Armor, Mystic Last Prism, Ruthless Lunar Portal Staff,

Arena/Strategy Ideas
Flat arena: The intended arena, a simple flat one, works reasonably well. Attempting to block enemy movement will usually just result in enemies phasing while your own weapons are blocked. Having Conveyor Belts along either side of the Crystal stand would greatly cut down on the need to collect Eternia Crystal / Hearts / Stars without impeding the placement of sentries. Also building the floor with Asphalt Blocks may help you run faster from side to side.

Wide arena: If there is enough room (no obstacles on a wide enough arena), there will be 118 tiles between the middle of a Mysterious Portal and the middle of an Eternia Crystal Stand; and 235 tiles between the middle of both portals. Crystal stand is 5 tiles wide; portals are 5 tiles wide. That means the total width of the arena should be at least 239 tiles if you want the most time to defend the crystal, perhaps a bit more if you want to surround it by walls to prevent monsters from falling out of it when pushed back. If you want the arena to be symmetrical, the total width should be an odd number, such as 245 for example.

Short walls around the crystal: If you have a good way to strike around or through walls (such as with the Mushroom Spear or Solar Eruption), put up three block height walls around the Crystal. Three blocks are the max height the game will allow before the crystal will reject your arena. This will provide a bit of a buffer against incoming projectiles, such as those from Dark Mages, Ogres, and Drakins.

Fall through arena exploit: An easy way to exploit T1(and slightly T2) is to build a 121 block wide platform, the middle 5 blocks are 1 block thick and hold the stand, while the other 116 blocks are 2 blocks thick and can be toggled (Active Stone Blocks or using Actuators). At each end of the platform, build a wall 4 blocks or higher. About 8-10 blocks under the platform, make sure there is a one block thick strip of land, spanning both directions. When the event starts, flip a switch to deactivate the platform. All enemies will fall be put into ghost mode, but for some reason, cannot make it up to the crystal. This allows you to easily deal with all ground troops, since they cannot attack. You will have to figure out how to deal with flying enemies, however.

Portal Warp: This arena/strategy requires you to unlock the Steampunker, in order to get the teleporter. First of all, build a flat arena, and preferably wide. Locate the position between the Eternia Crystal and the Mysterious Portals, and place your personal teleporters at the position that is halfway between them, connect them with wire, and place means of activating them. Now jump in! You should be circulating between the 2 portals, every time you activate them. Now use a piercing (preferably homing) weapon, like the Nebula Arcanum, and spam it towards the enemies. It should be able to beat all tier waves, up until wave 7 in tier 3. This can also be done with the Portal Gun.

Equipment
Weapons with high knockback work well in conjunction with Explosive Trap sentry summons.

An easy tactic to beat this event is to buy the Ballista staff from Tavernkeep. Every time after the wave has ended put more sentries about half way from the crystal and portal. Ballista staff sentry pierces enemies, so they hit many when they shoot. If you have enough defense medals, buy flame burst staff and install two of them behind the ballista sentries to make some AA fire. Stardust dragon or similar seeking minion is best for this event as he cleans your immediate neighborhood if any flying thing gets past defenses.

Arm yourself with ranger gear, S.D.M.G preferably, stand with the crystal and just shoot Luminite Bullets to each direction every once in a while, they pierce all enemies so they hit everyone. When Ogres or Trolls are seen in the map, shoot long enough to destroy them. After the sixth wave, change from Luminite Bullets to Chlorophyte Bullets and just fly above the crystal and kill the Boss. After 6 waves you should have about 5-6 ballista sentries & 1 Flameburst sentry summon tower on each side of the crystal so you kill the boss faster than enemies get to destroy the crystal. If you are worried about flying enemies, just clean them up once or twice close to the crystal before resuming to shoot the boss.

Above tactic requires late game equipment to work properly. With Vortex armor and a Ranged damage source, this becomes a very easy event.

A similar tactic can be used with Mage armor, as the Last Prism cuts through the event like paper, as the high damage, range, and piercing ability can get through the event with ease. the Flameburst sentry summons are useful with the last prism as the splash damage works in synergy, making your defenses explosive and deep cutting.

If player prefers melee gear, Meowmere with Solar Flare armor can easily destroy waves 1-6 without even moving. When the Betsy appears, Terrarian combined with Cosmic Car Key easily kills her. The Solar Eruption is also very effective.

A set of Tiki armor with a Stardust Cell Staff makes the third tier nearly trivial, as the cells can target enemies on both sides of the crystal, and will later focus Betsy.

Mana storage exploit
It is very helpful, especially in Expert mode, to set a chest nearby and stack any leftover Etherian Mana, since these mana will not disappear if you do not open the chest outside the event.