Alway`s mine the second level of the tunnels, you`ll save on # of mines that way, and won`t have to worry so much about people running around and putting mines everywhere!And as the black hand stealth soldier, don`t reveal yourself and shoot at your target, your gun really isn`t that powerfull.  Just plant C4 on your unsuspecting victim!  Its much funnier!
And if you play as a sniper, keep moving around!  Don`t ever stand still, playing as any character.  Do YOU want to be the next target of the sniper that does run arount a lot.  I DON`T THINK SO!!!
Enjoy
Peace