why does the god mode just show up while your doing the campain
Because its a computer controlled opponent, not another player
Regarding beating Godmode;
Obvious choice is knockback, if you have capacity to knock him back two slots per turn (one weapon with 2 knockback or two weapons with 1 each) he cant get to touch you.
Second easy choice is over heat, (as suggested by Aeacus in an earlier post).
Other choice if you cant knock back enough to do the above, is to hit him and keep backing up as he moves towards you, when you get backed up to the end of the stage, jump over him and repeat back and forth. He will get a hit on you at each end, but you should be able to kill him before he gets you. When you move back, go in large as possible steps so you spend less turns moving and more turns shooting.
If you cant knockback or jump (due to having wheel-type legs or heavy legs) or overheat then its a lot harder. If you can't do a single knockback other than stomp then when your backed against the wall, hit him with your sword (FFS you do have a sword, right
) and stomp - he'll only get one hit on you as he will move forward then hit but you'll get two hits (well a hit and a stomp).
I have defeated Godmode using all these methods.