or maybe e sentry-robot-thing or a armor that you can make for yourself

I know! There is a Garry's Mod worskhop model called the Deadnaut. That could be made, and would probably cost 1000 metal. It moves slow, yet is heavily armored. Be sure to keep Lvl 3 sentries and Soldiers and Demomen off it though, it has a little soft spot for explosions. (Literally)

I say, for the Deadnaut, you have to build it with 3000 metal, as its a mech
You start of by placing the base, cost 100 metal.
Then you continually feed it metal until it reaches level 4 [3000 metal, as it cost 1000 per level]
Once its level 4 you can titanfall it up.
Although weapon would be restricted to maps like upward
And would have a small spot where it would take 1000% increased damage
If it blows up, you and all nearby buildings you made are destroyed
all other buildings are 50% less effective.