One of the big challenges with 'difficult bosses' is the environmental factor. Go back and read the last couple of pages of this thread; they're full of tips like this (no offense intended to the author):
I dunno what problems you guys are having with the Golem. I just set up a wall thing so that there's a large gap but not large enough for him to walk through, then I'm up on an incline behind it and I just murder his ass with a tacshot and chlorophyte bullets. Didn't have any problem at all.
Basically: if a boss is too hard, screw with the terrain until he's trivial. If a boss becomes a puzzle boss, then people will just resort to 'solving' the boss by constructing arenas that mess up its AI or nullify its attacks. If the point of Terraria is to construct arenas that nullify each boss, then so be it; but in doing so, you've made it impossible to design bosses that can be beaten through creative use of various weapons. I guess terrain advantage is inevitable when the game engine allows you to destroy/replace blocks, but right now every boss is simplified or trivialized by the right terrain; the only useful aspect of weapons is their DPS (and their piercing, which influences their DPS), along with a small mention of their ammo cost, and I don't see a simple change that would fix this.
It doesn't help that there are really only 2 viable arena designs. If the boss passes through walls, then you need a long, flat platform so you can backpedal and shoot ranged weapons at him (Wall of Flesh, the Twins, Eye of Cthulhu, etc). If the boss does not pass through walls, you need a wall that stops him attacking you, while still allowing you to shoot him (Golem, King Slime, etc). Pretty much anything else is just making your life more difficult.