Another trophy you can get about 10 minutes into the game if you’d like. All you need to do is toss a grenade at your feet and wait for the explosion to send you into Fight For Our Life mode. Depending on your health and shields this may take a few explosions. You can also use any of the exploding elemental barrels littered around Pandora or a Tediore gun (Tediore fun’s are tossed out and act like a grenade whenever they are reloaded).
It’s worth noting that you need to play through the story up to the point where you rescue Claptrap and meet Lillith as this will unlock grenades for you to use.