the teleport loop
Casters never walk, jump or fall. Their whole existence is a four-beat loop: find a legal spot, blink to it, fire three shots at you, then stand still for about three seconds and start again. The spot has to be within 20 tiles of you but no closer than 4, has to be a pocket of open air at least four tiles tall and three wide, and cannot contain lava. Learn that rhythm and the fight becomes a timing problem instead of a chase, because the three-second pause after a volley is the only window that matters.
walls do not help
Caster projectiles travel straight through blocks, so the instinct to duck behind terrain gets you killed by something you cannot see. The one exception is the Necromancer, whose shots stop at solid tiles. Everything else, including the Dark Caster and Fire Imp, will hit you through a wall you just spent a minute building.
Some of those projectiles are fragile. The orbs thrown by Goblin Sorcerer, Tim, Dark Casters and Fire Imps pop after one connection with anything you happen to be holding, tools included, which is why swinging at empty air in their direction works so well.
damage locks them in place
Hitting a caster mid-volley cancels the remaining shots and starts its pause early. Hitting it during the pause resets the pause back to a full three seconds. Keep landing hits and it can never fire and never teleport, which is the cleanest way to kill one. The catch is per enemy:
- Necromancer and Diabolist blink away the instant they take any damage, so a weak weapon just chases them around the room.
- Ragged Caster does not flee on hit, but it fires the moment it lands with no wind-up, so the usual reaction window is gone.
- Anything else can be pinned indefinitely by a fast enough weapon.
blocking the shots
A weapon that keeps swinging while you hold the button doubles as a shield, since each swing deletes any projectile it touches. The Muramasa does this well, and a pickaxe or axe works nearly as well because it also autoswings. An Inferno Potion covers you without any input at all.
Better still are weapons whose own attacks pass through blocks, because they clear incoming shots early and hit the caster on the far side of the wall at the same time. Death Sickle, Nettle Burst, Mushroom Spear, Chlorophyte Saber and Crystal Vile Shard all do both jobs at once.
where they can and cannot go
Dungeon casters can only blink into spaces backed by naturally generated dungeon brick wall, which effectively chains them to the dungeon. That is exploitable: clear out a room, replace or remove the background wall, and stand in it. They cannot follow you in, and the ones that shoot through blocks will still be aiming from wherever they were stuck, which gives you a fixed firing line to plan around.