You have to complete the Malevolent Crucible in the Alisha DLC in under 2 minutes to get this trophy. You can set the difficulty to simple. Use the right stick to command your partner to spread out. I recommend controlling Alisha and using the long range arte Dragon Fang when the slugs come out so you don't have to walk up to each one. There isn't much else to be said except that you need to have pretty high stats to kill things quickly enough. This is very difficult to get if you just hop into the DLC straight from completing the main game. However, if you come back on a NG+ playthrough with the double damage bonus skill from the grade shop, it becomes a very easy trophy. To do it on a first playthrough, you're probably going to need to max your levels and have very good gear.