Spectre armor

Spectre Armor is an extremely effective late-game mage-style armor crafted from 54 Chlorophyte Bars and 54 Ectoplasm. Its set bonus grants healing upon successful magic damage above a certain threshold. Every time the player wearing the armor causes magic damage to an enemy a small white sphere is created that travels to a nearby team member who has the lowest health and heals them for about 8% of the damage dealt. The orb will also heal the wearer, if no team member has lower health or if playing a solo game. Each sphere heals only a small amount of health, but any cast that hits multiple enemies will provide health upon every hit and certain attacks can quickly add up.

Despite its low defense and moderate resource cost, it is still considered to be the best mage armor in the game due to the massive damage stats and the very handy life leech.

Tips
Items like the Shadowbeam Staff and the Magic Dagger are a good combination with the Spectre Armor, as the healing effect will continue, providing you have enough mana.

Magic Cuffs are a great accessory to combine with Spectre Armor, due to the effects of dealing and receiving damage healing your health and mana, respectively. The player would take damage enough to heal upwards of 45 mana from even a regular zombie, meaning that potions could be all but eliminated from use during late-game boss fights.

Using this armor, as well as a full set of Menacing accessories can cause a player to deal massive amounts of damage, over 110 a hit with the Inferno Fork.

Multi-hit spells like Inferno Fork, Demon Scythe and Magnet Sphere work extremely well against hardmode bosses like The Destroyer and the later hardmode bosses like Golem and Plantera. Keeping your teammates as well as your self healthy throughout the fight.

The Golden Shower works extremely well because one click will deal piercing damage three times per click as well as lowering the target's defenses, thus allowing you to deal even more damage and heal even more. A player equipped only with Spectre Armor can deal about 144 damage per 4 mana, healing 14 points of health from a single enemy. That means for every "star" of mana you spend, you'll heal over 3 full "hearts" of health.

Casting long-lasting spells like the Nimbus Rod is a great way to keep your party healed, as each raindrop counts as a new projectile, healing the maximum possible health. If an enemy is trapped underneath a Nimbus Cloud, health can be restored while completing another action.

Combining all these techniques together, one can decimate even the toughest of Hardmode bosses. For instance, using the Golden Shower and then attacking with other, more powerful, weapons will quickly deplete bosses' health. Combined with the Magic Cuffs, one will almost never run out of mana or health and can attack the enemy risk-free, making short work of it.

Trivia

 * Spectre/Specter is another word for ghost or spirit.