The Wave Concussor is a very effective gun to take out many ground enemies all at once. The fully charged attack has a big blast radius and is perfect for when you are surrounded. As a downside, it's a bit impractical if you're surrounded unexpectedly (due to the charge time) so charge it up beforehand if you have ammo to spare.
This time, you're pitted against thirty marauders who will arrive to attack you one after the other. They still only have their leaping slash attack. Not to mention, they might also begin to fight each other if you are too far away. The arena now is also four times larger than before, with each platform being submerged in lava every so often (there'll be a loud warning noise). The best course of action is to just charge up the new Wave Concussor and fire it, although the Blaster's spin plus fire combo is good too.