Beat UNDERCOVER with everyone in the crew only using the GL40. To complete this challenge, everyone in your crew will have to have played the heist from the start. Using melee does not count towards this challenge!
After unlocking the GL40 (lv.21 of the Technician tree) go into UNDERCOVER. This trophy can be earned on any difficulty, just make sure to have at least one friend and run ammo bags to make this trophy even easier. Also, as the description says, be careful not to melee anyone!
A tip from DarkSeraphim666:
I just went for Blow Out and used my pistol while I was in bleed out and still got the trophy. Also, only people who are alive at the end will get blow out.