My DS is almost 13mo and is a pro at the "hip thrust, crocodile roll" maneuver. My DD was the same at this age, I think it's pretty normal! I usually dress him in the hallway after a bath because DD is still in there. I typically end up chasing him up and down the hall! I get the nappy on first then let him crawl around, sneak the singlet on when he sits up to play with something, then put his PJs on while he stands against the bath giggling at his sister.
For nappy changes, I use distraction as much as possible. A small toy usually buys me just enough time. Otherwise there is a game I play with him where I say "I'm gonna eat your..." With a long pause then I say "feet!" Or "belly" then pretend to do it. He's used to the game now so when I start saying it he stops wriggling and smiles, anticipating the tickles. Is there a little game he's familiar with like that?