The online pvp is accessible via the conflict symbol next to the white house. I would suggest to be part of a clan before going for this, to get 2 achievements at once. The only 2 game modes available are skirmish and domination. Matches last 12 minutes, unless you finish the objective sooner. Winning can be tough, but should not take too many tries. I suggest to run with the drone and the turret and try to work as a team. If you attack 1 guy with 2 of you, you are more likely to kill him than vice versa. Also not sure if you need to be in from the start. For the clan battle achievement you do