how do you get such a significant boost with the fire extinguisher?

i’ve been doing some runs recently and i’m having always losing time on level 7 because of how much i have to spam that r2 button to clear the onslaught of enemies. so i would like to try and find a skip for this level but i may need some help. i’ve found out that you can just simply walk through the exit box and you can jump to the right side of it and have cover whilst being able to shoot the on coming enemies, this is great but it takes time for them to come to you. i’ve been trying to see if there’s a way to trick the game into thinking you’ve met the kill requirement for the exit to work but i haven’t had any luck, maybe one of you could help?