I did:
3x First aid 2x juggernaut
3x hostile takeover 3x nav crash course
3x furious charge 3x xp
3x annex
bounty hunter
but taking planning is a good option as well
hostile takeover and bountyhunter capitalize on his bonus xp if he did not have that, i would have taken planner
My skills for Boris
3x First Aid, 3x Jog (he's used a lot, so lots of walking)
2x Navigation and Overrun-2 (yes, some Boss camps are a good target for Boris)
3x Charge, 3x Fast Learner (both quite essential combined with his special skills, unless you're at/close to lvl 75, then no need for FL)
and Frenzy-1
3x Annex
1x Master Planner
Well, I took frenzy to use Boris vs. Steersmen camps, but if your Mary has both Overrun-3 and some books in Frenzy, she's a better choice there.
Frenzy may also be useful in some MCC camps (possibly with more than 1 book as well), but I avoid MCC for now
Navigation: because my main gens all have Navi-2, so just to keep it simple
my skills for boris are
2x First aid 2x jog
3x Overun, 2x hostile takeover
3x unstoppable charge 2x xp 2x Battle Frenzy
3x Garrion annex
1x master planner
Thanks for your answers! Always good to hear input from others before deciding for yourself.