I'd use Steel-Types. For no reason. Because there's not enough steel types, I'd add some Grass types? ..
Aggron
Metagross
Cacturne
Breloom
Tropius
Perhaps?
Well. I haven't played Pokemon in a really long time. Now I decided that I should come back to it because I have nothing better to do. But if I do I'd have to build teams and stuff. I have no idea how to build teams what so ever. I really don't want to read these long tutorials. I don't get it...