I had a lot of trouble unlocking this Trophy, I managed to unlock on my third character creation. Here are my tips:
1) Make sure you hold the store up, rather then punching the cashier and stealing from the till. Once the cashier has handed the money over the counter and you have collected it, shoot the cashier in the face. You will get a minimum three star wanted level, but there's no worry of the cashier shooting and killing your character. If you die in the shop, this can still tick a shop off but also disqualify from getting the trophy.
2) Hold all the stores up in one session, I think the cloud might have some issues saving certain stores if you hop from one session to another session. Holding all the stores up in one session should help to eliminate this issue. Also recommend an invite only session, this will stop random players killing your player.
3) Have an escape plan before you rob the store. I recommend using the underground tramlines if robbing in the city or the train tunnels in the desert, make sure you set your GPS beforehand.
4) Make sure your character is healthy, buy some snacks from shops and get full health before robbing, Also, when you are driving away from the coppers, holding the middle touch screen button, select inventory, then snacks and have them ready in case you crash your vehicle with trigger happy cops behind. This can save you from certain death.
5) Rob ten stores without a mask, and ten stores with a mask. Just makes it a bit easier.
6) Rob stores closer to an exit plan later on and rob stores further away from exit plan earlier. Makes sense to rob the more difficult stores with a lower wanted level.
7) Use a decent car, I recommend the Vapid Sandking, its a car that has a lot of weight, easy to manoeuvre and can go offroad with relative ease. Make sure the car is healthy before robbing a store as well.