Higher damage stats will definitely help.

Also, try to attack the enemies from a distance. The crosshair allows you to target tangos well beyond your screen's borders. And the guns are all pretty precise, even the pistol.

Just zoom your camera out and snipe enemy before the can see you. You will be able to do all levels solo if you just zoom out

Joining this party late, but I just played this round today, so:

I have tried it in different tactics, the best was to go in and kill all the bad boys, before first wave is starting.

Then you can just destroy the bandits and slots and when waves are coming you only have to get those and no random walking enemies.. But I really hope you have already found a way. ^^

Have fun

yes you want to max out both characters first level is the best/easiest/quickest to earn the top amount of xp.

unfortunately there seems to be a problem for 99% of people at the moment when you complete the casino level it doesn't show up as complete all the stats are well out of the window basically uncompletable although very few people have managed to complete this.