LoM2
Retinazer
Lieees my water gun does nothing to itKing slime, the only boss killable with no effort, with any gear and any weapons, just after placing a few blocks.
In all seriousness, KS is the easiest... He's basically a slime with a lot of health.
In terms of HM: The Golem would be the easiest. Literally takes nothing but standing next to him swinging a sword.
(I had a lot of trouble with the Destroyer until I got a Nimbus Rod... Plantera is kinda wierd, first time I beat her, second time I died, third time died, fourth time beat her, etc.)