You'll be able to use the Scatter Gun in this mission, a great short ranged weapon that has a large cone of fire with a good knockback as well. The marauders (twenty total) will appear one by one from the cages on the edges of the arena and can use the square boxes to hide from you, so be prepared to watch your back. They have one attack here which is to run at you and take a short leap before they come slashing down.
The trick is that the arena's lava will rise up from time to time, forcing you and the marauders on the boxes for safety. These same boxes also dispense crates with ammunition. Eventually, after 15 kills, you'll automatically transform into Dark Jak; use this opportunity to quickly and easily get rid of the remaining five marauders and win the challenge.
When you exit the arena, you'll automatically meet Kleiver who gives you your next mission.