First Huge Wave: What the Hell Do I Do?
Basically you have to micro your hero a lot. Go out in front and stutter step backwards. When they begin hitting the wall, dash behind and shoot them from the rear.
Also, this is not a chill game. It will NEVER be a chill game until day 24 or whatever and you are basically maxed out. It is a frantic relay race with some metal slug shooting.
What got me through it was having 3 upgraded walls facing the left. Depending what comes in the wave you have to thin them and fall back, if the little bomb guys are there in numbers the walls are down almost immediately though.
If you have the choice for the brazier upgrade that burns them if they pass it, that helps a lot too.
Getting behind them when they hit the wall can also radically change things but is very risky.