I'm amending my previous review, I was too harsh. I rated it before having played the game much, which I should not have done. I really enjoy the game, which does not deserve a 3 star rating.
So, this is definitely not an easy game. And it relies a bit on luck. When you level up, you're presented with 3 powerups to pick from. You're at the mercy of whatever the game gives you. I stand by the oil leak being the best powerup by far, as it essentially shields you from weaker enemies. If RNGesus gives it to you early, you're going to last longer because you will kill pretty much everything at the beginning, earning you more EXP, more levels earlier, and with that more weapons and whatnot.
I made it sound like the other weapons aren't useful. That isn't true. While of course some are better than others, you can manage without oil leak if you smartly choose. While I don't think luck reliance is good game design, it works here. This game is about working with what you're being given. It takes trial and error to figure out what you should prioritize. I do still stand by the frequency of enemies being too high. But overall it's fun. I also appreciate that the ad revenue is being put toward a good cause.