beat story mode first, get the feel of it and play co-op online.
story mode: get all towers first then all outposts. crouch 75% of the time cuz animals WILL KILL YOU if alerted, get silencer on all weapons available
co-op: all about team work, stick in same area covering dif. points. comunicate with your allies, if your down let them know how much time they got left to revive you....dont pressure them to revive you without securing the perimeter first. oh and try not to rambo your way through a level, you will get killed.