If you won't be using Assault class for normal operations gameplay you can grind this out at the Armouring Hall. The third trial is called Death from Above. As soon as it starts hold Y to boost in the air and then hold in RB and aim the ground pound at the group of melee Tyranids that spawn in front of you. I would do it twice and then run to the back of the room and fall off the ledge to kill myself. Hit Y to restart and you can prob grind this out in 20 minutes.