Solo Easter Egg Liberty Falls Advice

I’m going to attempt the Easter egg for Liberty falls after work today. I want to do it solo, and I have all the parts memorized. I’ve gotten to the second wave on round 33 with some randoms online, but all they dcd at that point. My pack a punched ray gun and pack a punched jet gun literally did no damage to the elite mangler on wave 2. I’m just looking for advice mainly for the final battle with the 3 waves and how to survive it solo.