If you don't know what a Spider Tank is, I suggest you check this out.
http://tvtropes.org/pmwiki/pmwiki.php/Main/SpiderTank

Anyhow, this Puppy is a Spider Tank that is armed to the teeth. Missles, Guns, Elemental Bullets, Flames, Oil jars... etc.
It should also have turrets that shoot things similar to charged Alchimer shots, which may split into two (similar to lower level alchimers) or four (higher level) shots. They can also bounce around the room in predetermined paths unlike the Alchimer's randomized bouncing trajectory.

Spider Tank Puppy also borrows some of Turtodrone's attacks, such as launching electrified mortars straight into the air and going down into specifically marked areas.

It should also have Area of Effect attacks similar to how our bombs work.

Unfortunately this expensive piece of equipment is incapable of fighting close quarters, and will heavily rely on various stationed Normal and Elemental Gun Puppies and more mechanical themed enemies.
Also, destroying certain parts of it (like a fuel canister for its fire attacks) will permanently disable some of its attacks and do severe damage against it.

As it dies, it should explode with the force of a nuclear bomb (without the fallout of course) and you will instantly die if you don't make it to cover in time.

Also, I think there should be conveyor belts, that automatically move the player in a direction. Heck, the boss room should have conveyor belts.
Crushers as well. Not just from the top, but on the walls, crushing players against another wall.
Spikes on a wall too. I mean, it wouldn't be any different than thorn bushes against a wall, but it would be nice to see one of those.