Found a spot to gain ACE and XP easily? Maybe guys are constantly dribbling down the fast lines and you can shoot them easily, or a bunch of terrorists are conveniently bunched up so you can kill them with a sack of C4.

Simply perform the actions you want for ACE and XP, then before you hit the next checkpoint, let one of those wannabe terrorists kill Bishop or Knight, forcing you to reload the checkpoint. You keep the ACE and XP you acquired (just don't turn off your system) and you can re-do that section again for more points.