The combat mission objectives is planting a bomb and detonating it. I believe it's ONLY if you plant the bomb, and ONLY if that bomb detonates it. If the bomb doesn't detonate (for any reason) you don't get credit.
Do this 3 times and you'll earn the trophy, overall it's not hard to get this, just focus on trying to plant the bomb over earning points and you'll get this as you're clearing other online trophies. As combat mission is about the only online mode that is being played, you should have opportunities.