This trophy is not as hard as it sounds. If you're seen, you can simply revert to the last save and it'll continue to count as a success. It's best to find a level that has no vehicles or a smaller number of enemies. I personally did Halfaya Pass (I had to lay down 3 mines in the same spot to make sure the Panzer died without having a chance to spot me). As long as the enemy's circle doesn't turn full red, you're fine. Full yellow, and a little red is fine and will not void the trophy. Just be sure to revert if any enemy goes full red or you think they fully spotted you (Including snipers).