What do you accomplish by grappling the tanky fighter-type? Making them stay next to you until you're dead? That's probably what they were planning to do anyway.
I'm going to assume that by "trip," you mean "shove."
1. Athletics check to grapple, can't move.
2. Athletics check to knock prone - grant advtg & disad to hit.
3. Profit.
#3 Assumes the GM rules the prone target cannot stand while grappled (unless they first win a grapple roll), which is certainly how I'd run it.