RESIDENT EVIL 5 CO OP Group :)

to beat this part by bypassing that entire part of the battle (ie: Not having Sheva advance until Chris had pushed the boulder out of the way.

How I did it was as follows:
(Obviously I'm playing Chris)

1) After you fall, Chris runs as quickly as possible straight to the boulder.
2) While Wesker is slowly following Chris, Sheva shoots him a bit to slow him down (but never enough for Wesker to chase Sheva to the ledge).
3) Chris punches the boulder out of the way as fast as possible. If you're doing this right, then Wesker will still be pretty far from Chris by the time the boulder is down.
4) Sheva joins Chris by crossing the lava. You'll notice that when the boulder is removed, Sheva is automatically moved to the point where she would have climbed up had she fallen and done the grab-onto-ledge QTE. That means you never have to deal with the Sheva button mashing part.
5) Chris and Sheva both hit Wesker as he approaches (I was using a Grenade Launcher with Explosive Rounds). Eventually, he'll do his animation and jump to the ledge where Sheva came from.
6) Run to the high ground and the next phase of the battle will begin.