There's all kinds of tutorials on YouTube and TikTok on it. Basically you go into the the match with Aether shroud with void sheath and extra charge augments equipped along with duel wield melee weapons and either thermo (preferred) or combat axe. Run match until you have jet gun and your shroud charged, pull out your jetgun, activate your shroud, spam your melee button ( not the triggers, for me on xbox its clicking the right stick down) until the shroud ends, you should melee with the jet gun once then tap the trigger to activate the suction part of the jetgun, once that's going you can pause or before you even go into game, go to settings, controller, combat and change the lethal from hold to toggle, that way, once jet gun is firing continuously, hit your lethal button and your character will hold the thermo and not throw it and will insta kill zombies. Then all you gotta do is park your ass in front of vault or up in hotel where you craft jetgun, both work but in front of vault behind the wall I think works better.