This guide describes how to complete 100% of the Bestiary in one world without bringing anything from another world. It assumes you have gone through a classic walkthrough, that you are in a world created in 1.4 without special seeds.
At the early game, you shouldn't need to worry about completing the Bestiary. You'll likely have completed multiple entries already, mostly Town NPCs, critters, and common enemies such as Slimes and Zombies. Your lack of power or wiring equipment, however, make it more difficult to farm at this stage, so don't stress about filling in the Bestiary in early pre-Hardmode.
Checking for Rare Creatures
Some rare creatures are even rarer in Hardmode, so you should beat them during pre-Hardmode. You can increase your chances of seeing them by using a Water Candle / Battle Potion / increasing the spawnrate in Journey Mode:
- Giant Worm (167) in Underground layer
- Nymph (181) in Cavern layer
- Tim (184) in Cavern layer
- Ice Bat (211) in Ice
- Doctor Bones (243) in Jungle at night
- Devourer (312) in Corruption
Getting the Other Evil
Now that you are ready, you can beat the Wall of Flesh and go in Hardmode. To complete the bestiary, you will need to get some Crimson in your world if you are in a Corrupt World or some Corruption if you are in a Crimson World, but it is only doable once you are in Hardmode.
To complete the Bestiary, you will need the other evil in your world. To do that, you need to gather at least 6 Tombstones created after your deaths (or after your NPCs' deaths if you are in Hardcore). Place the 6 tombstones around the Dryad to buy some Corrupt Seeds/Crimson Seeds from her (you must be in Hardmode). If you didn't beat them already, be sure to kill some Ghosts, Maggot Zombies and Ravens, which spawn in Graveyard biomes. On the Desktop version, Hoppin' Jack also spawns in a Graveyard. You will also see some Rats while in the Graveyard biome, and Maggots will spawn after killing Maggot Zombies; simply walk near them to complete the Graveyard part of the Bestiary.
Other Evil Surface
Now that you have some evil seeds, plant them in a place where there is a lot of pure grass. After some time, Vile Mushroom/Vicious Mushroom will spawn, which you will want to gather. You can then transform them into Thrown Powder to spread the evil even more. If you prefer Thrown Water, covert some sand with the powder so you will be able to make some. Before leaving the biome for now, summon then kill the Eater of Worlds/Brain of Cthulhu.
Completing the Biomes
Now you should have enough supplies and should be able to complete the different biomes. Be sure to get a Clentaminator, some Green, Blue and Purple/Red Solution and some powder/water of the other evil.
While you are on the surface, capture two Penguins, and kill monsters at night and while it is raining until you've seen the 7 critters and monsters (numbers 92+). Once you unlocked the entry for all of them, go underground. Use your Clentaminator with green solution if the Ice biome is not pure, then start throwing your powder or water around to corrupt the biome with the other evil. Wait around until you beat Pigron (purple if it is Corruption, red if it is Crimson). You can then use the other solutions to transform the biome again and kill the other two Pigrons. If your underground snow bestiary is not complete, spend some time there with a water candle and battle potion to beat them. An additional (and maybe easier) way to defeat the pigrons which does not require making your underground snow biome corrupt, crimson and hallow is finding a pigron statue. Pigron statues will spawn purple pigrons, red pigrons and pink pigrons.
This part should take some time as you will need to convert a desert into 4 parts: one Hallow, one Corrupt, one Crimson and one pure, then wait for a Sandstorm. There are four kinds of Sand Sharks, one for each type of desert during a Sandstorm. You will have to beat each in order to complete the Sandstorm event in the Bestiary. You should also beat each type of Mummy this way. Now the Desert, Sandstorm, Corrupt Desert, Crismon Desert and Hallowed Desert should be complete. If not, you should wait longer in this area or put some water in it if number 96 or 97 are missing (these are oasis critters). Once you are done, go underground and free the Golfer if you haven't already. Then do the same as was done for the Underground Snow biomes by creating corrupt, crimson and hallowed areas. There is a ghoul for each to beat, and you will have to beat a Lamia in Corruption/Crimson and one in Purity/Hallow. After that, you should have completed the 4 Underground Desert bestiary sections.
The rest shouldn't be hard and by now you should have completed most of the Bestiary:
- Windy Day
- Rain (if 338 (Rainbow Slime) is not unlocked, go to the Hallow and wait while it is raining)
- Underground Mushroom (except 91 (Truffle Worm))
- Spider Nest
- Snow (except 23 (Cyborg) and 25 (Santa Claus). Make sure you have unlocked 22 (Tax Collector) by purifying the Tortured Soul!)
- Underground Snow
- Underground Desert
- The Underworld
- The Corruption
- Underground Corruption
- Corrupt Ice
- Corrupt Desert
- Corrupt Cave Desert
- The Crimson
- Underground Crimson
- Crimson Ice
- Crimson Desert
- Crimson Cave Desert
- The Hallow (except 109 (Prismatic Lacewing) and 507 (Empress of Light))
- Underground Hallow
- Hallow Ice
- Hallow Desert
- Hallow Cave Desert
- Surface Mushroom (except 21 (Truffle))
- Goblin Invasion
- Pirate Invasion
If a creature isn't unlocked yet, go to the biome (or create it) and spend some time in it.
Now you have completed the different biomes, you can continue your progression and beat the others Mechanical bosses if they aren't already dead. Capture two Bunnies and two Goldfishs, take three evil powder of each kind then convert one Bunny, one Goldfish and one Penguin to each evil by throwing one powder at them, then kill them. It is then time to fish. Summon a Blood Moon with Bloody Tear or wait for one to happen and begin fishing. You will have to kill at least one Wandering Eye Fish, one Zombie Merman, one Hemogoblin Shark, one Blood Eel, one Dreadnautilus, and one Blood Squid from Dreadnautlius to complete the Blood Moon part. Use the Chum Caster as soon as possible to fish them more easily. A Sonar Potion will allow you to see if you are going to pull up one of these enemies.
Now you can continue your progression by killing Plantera, Empress of Light, Golem and Duke Fishron. Complete the Dungeon, the Temple, Underground Jungle, the Solar Eclipse (during it, let at least one mothron egg hatch to kill a Baby Mothron), Ocean and defeat the Old One's Army. You must also fill out the Martian Madness by finishing one event and killing one Martian Probe. Note that the Martian Probe spawn chances are highly increased when you are near a Water Candle.
With your new gear, you should be able to beat the 15th wave of the Pumpkin Moon. Once it is done, you will have one day of Halloween. Alternatively, set your device's date to October. There are 11 creatures to unlock: go directly to the Caverns, increase the spawnrate because you will have to kill three different skeleton in costume, which are quite rare. Once they are beaten, go in the surface again and place a cemetery if it is day. You will have to beat one special type of Bunny, one of Slime, two types of Demon Eye, three types of Zombie, and Hoppin' Jack which spawns at night (though it can also be found in a Graveyard on the Desktop version).
This one will be tougher. You have to beat the 15th / 20th wave of the Frost Moon, which will unlock one day of Christmas (or set your device's date to December). Once the Christmas seasonal event is active, beat as many monsters as possible. There is one type of Bunny, 4 types of Slime, and 2 types of Zombie. Open as many Presents as possible, as you will have to quickly start and beat the Frost Legion. If it is still day, Santa Claus may spawn. Otherwise, you will have to beat the Frost Moon again to make him spawn.
The only monsters remaining are linked to the Lunar Events.
By beating the bosses normally, you will miss at least a monster. While fighting against the Lunatic Cultist, you will have to either hit a duplicate Cultist or not attack them when they are in circle once to summon the Phantasm Dragon, then again while the Dragon is still alive to summon the Ancient Vision.