The first mission you can get this trophy on is mission 9 back up, back down. Its a simple mission for the trophy too. After selecting your landing zone your prompted with the mission loadout menu, go to character select and change from snake to anyone else and make sure to equip the grom-11 for this mission to blow up the trucks then start. All you have to do is using the grom11 destroy at least one marked vehicle, if you just want the trophy just destroy 1 and hide until the 15 minute clock runs down or if like me it is your 1st attempt at the mission go for as many targets as you can get... Hope this helps.
