If you 2 are the first near the enemies, they attack you, not Su-25s, and if they try go after Su-25s, just go after them and their journey ends with R-27ET. Save all Su-25s and they should get the job done.
Edit.
my point was that I think it gets easier when you fly faster than intended (waypoint speeds), you'll be a lot ahead of Su-25s so the enemy never has a chance to attack Su-25s. So you have better chance completing the mission.