Try different play methods. I like to do a Speed Run every now and then, it allows you to consume the co-op campaign really quickly and get a nice exciting vibe from it. My fastest time on Rookie is 1hr12m. Rookie is good cos you can just push right through the entire campaign with no restarts or mission failures. Heavy use of M&E increases speed drastically.
The other method I love is Ghosting. You can run through 3/4 of the campaign without shooting any lights at all and only have to kill a minimal amount of guards. I think the lowest I got prior to Mozdok was a mandatory total of 6 kills and 4 ko's.
Check this thread started by a real awesome dude, it goes a bit more in depth. http://forums.ubi.com/showthread.php...O-Forums/page3
(I don't play SP at all anymore, CO-OP is significantly more satisfying, it adds that extra challenge of managing a partner. Much fun).
I'm actually considering some co-op field work myself again, brush up a bit in prep for GRFS.