I tried myselfe some combinations of two weaponslot only (also included two mist bombs once. was fun but took Ages) but for now i would say that the Combo of DR+Wildfire fits best.

I use in the last Arena my chaos set plus sprite bomb damage buff, therefore i have max bomb damage here. This causes to kill almost each gremlin of a wave after i circled one half of the room twice while spamming bombs (maybe 1-3 gremlins run away with Little health Points left). In my opinion this is fast enough.

aside DR i use wildfire over polaris because of two things:
-wildfire causes an additional burning dps
-different to polaris, wildfire does never stop an enemy, so i can knock a crowd of Zombies or lumbers across the whole room, which usually is the end of said crowd that gets knocked. Polaris does shock and this causes two Problems for me: 1. Monsters can get a shock-tick right in the Moment i Need to shield bump thm away creating a wall of Monsters that causes death, 2. Monsters can get shock ticks, when i want to knockback them throgh a room, which would Need me to slow down, and Keep my distanceto said Monster otherwise i get too Close and the polaris bullets would not be able to expand while i run with full Speed.

i have both heated to Level 10: polaris and wildfire. wildfire i crafted heated exclusively after this Mission got published and when i was satisfied with the 4*-version's usefulness.
My first loadout idea was permafroster+wildfire, so i tested the two 4*-Versions of These weapons together. It is a loadout that can get the Job done, but DR proved much more relieable when it Comes to dps, plus it does not cancel fire. If it Comes to a defensive stance, the frost does actually hinder less about pushing enemies away. But if you want to run with high Speed into an enemy Group while Shooting, then ice stops enemies too aften to make it satisfying.

TL,DR: use DR as shadow weapon and i recommend wildfire over polaris.

^Not good advice

Last thing you want in that place is to get stuck in place for more than one second, or the lost souls will damage you. Also, gremlins don't die immediately, and block the bullets during their death animation, which prevents you from hitting the ones behind with the Grim Repeater

Polaris is great for the arena, but for your second weapon I would recommend Umbra Driver against gremlins due to high damage and mobility, unless you are very good with bombing. Perhaps a flourish to finish off wolvers as well.

Mobility is the best defense there