J.V.
The best way to deal with that is to recognize what he wants. So, if he is arching his back and refusing to get in the carseat, find out WHY. My daughter has done that a few times when she didn't want to go home. I would tell her I was sorry that we had to go home, but explain why we had to go home, etc. and promise her that we would come back to where we were, or whatever. If she continued, then I would just tell her that " I am sorry, but if you won't cooperate, then mommy is going to have to force you into the carseat."
I have a friend whose son loves to throw himself on the floor. When my daughter did that, I would just leave the room and ignore it.
It will get better, just hang in there and ignore as much of it as possible.
BTW, terrible twos peak at 18-24 months, not when they hit two.