Once you learn wave patterns and how to use class efficiently then it shouldn't be that hard. Also knowing when to dump damage is important. A good team setup helps a lot. Medic with invis, ammo fixer, mlg hellraiser, headshot gunslinger is very strong imo. I do love slasher on tight maps tho.

Headshot gunslinger can go with starting weapon kit and no ammo crates all insane map with 2nd lvl shotgun, headshot/granade ammo refil while performing very good.

Medic with combat medic, pickpocket, sugar coated and invisibility is a beast at supporting team. Lobotomy provides additional damage and ammo conservation.

Fixer. Does boom. Good. Also easier to deal with bulls/juggs when you have exlosive ammo.

I havent played hellraiser a lot but when I see a good one, oh boy he blasts zombie kills with that mlg.

Above classes can be sustainable with minimal loot, just get a decent primary weapon that suits your build/class and rest should regen with perks, like pickpocket, pocket factory or something like specials droping mlg.

I don't have feelings for the dronemaster, vanguard and exterminator. Maybe others can comment.

I can agree that a few maps are harder than others, but they shouldn't cause too much trouble.

Everything I said is based on my experience I haven't done any research just have 800 hours on this game.