For this achievement you need to have the most kills and be on a winning defense team. This doesn’t mean the best K/D spread, just kills. I’d recommend setting up a game with a 3 minute time limit. The less time the enemy has to finish the objectives the better the chance defense has of winning.
The best way to pick up kills is to mercilessly throw yourself at enemies with a shotgun or sub-auto weapon while throwing your frag grenades where you think the enemy may be. Obviously you shouldn’t expect to come out with a positive K/D spread but hopefully you might get off enough kills to be the “best” on your team. That or you could set up a match with a buddy, kill him for the most kills, and then win.