I think the boss focus of "can magically phase through walls" was extremely cheap, and precluded making bosses that were actually tough because then they'd be unbeatable.
So here's my "boss" proposal.
Description and Capabalities
A goblin enters from the left, riding a bipedal mech four times the size of a character. It has a machine gun, which shoots quickly and does decent damage, a rocket launcher, which works like bombs and destroys terrain, and a heavy cannon, which is like a slow machine gun with heavy damage that destroys a single line of blocks a few blocks deep when it hits it and can be exploded remotely, and moves very very fast for a projectile.
It also has a powerful melee swipe that destroys blocks in front of it, and a stomp that destroys the blocks under it's feet.
It can also jump and has jump jets for a double jump.
Behaviour
It will target players that closest to it with it's machine gun, unless they are in melee range (in which case it will swipe). It will use it's rocket launcher against enemies that are far enough away and will prioritize attacking clusters of enemies with it. If there are thick walls between it and it's target, it will attempt to get a distance from the walls and take them down with the rockets until it can path to it's target. It will also use rockets against thin walls with a target close on the other side.
For targets behind thin walls but a larger distance away, it will use the heavy cannon, and the machine gun can not fire vertically, meaning characters who attack it from above must be killed by rockets (slow moving) and the cannon (slow firing).
It will attempt to intelligently respond to and dodge incoming attacks, if possible.
If it can't path to a character and can't safely use rockets, it will attempt to dig to them with stomping and swiping. This is it's biggest weakness - if it can be lured underground, it's rockets can't be safely used and it loses most of it's mobility.
There, boss proposal. Tell me - would you rather fight something like that or the bosses Terraria offers now?
(and it doesn't cheat)