Good question. I remember attempting to replicate such a fight on a smaller scale back in my childhood using an empty coke can and my pet gecko- Henry.
Unfortunately they both completely ignored each other and Henry escaped into the garden.
BUUUUT I could figure out how it could have worked. See, the trouble with most robots is that while they're all high tech, well armed and highly defensive, they still lack the maneuverability, quick reflexes and instincts of it's lizardy foe. Giant robots are normally piloted by man who usually need to be experienced to be truly effective. Perhaps if this robot pilot had faced a Lizard before, he'd be ready and know the weaknesses and patterns of a giant monstrous lizard, but I'm considering this to be a first fight, so I don't think he does.
Thusly my money is on the Lizard.
