The "Giant 2nd Dimension Heinz Doofenshmirtz Robot" was a giant robot modeled after and built by Heinz Doofenshmirtz from the 2nd Dimension. He piloted this robot as a last resort effort in killing Phineas Flynn, Ferb Fletcher and Perry the Platypus, but his 1st Dimension counterpart managed to stop him before he killed them. Doofenshmirtz shows Heinz Doofenshmirtz the 1st Dimension Choo-Choo which Heinz owns and gives it to him, removing all the pain he had in the past. After this he destroys all the Norm Bots, and presumably this robot too, since It's nowhere to be seen after that.