This game has some pretty good potential, but there's some seriously annoying balancing issues.
As others have mentioned, the basic enemies in the final level have a 50% chance to block an attack completely. Even with the bazooka, it's down to random luck whether or not one of the enemies will get through. And the moment one of them gets through, you'll get mobbed utterly and you won't have a real chance to push them back anymore. It's quite annoying to lose in the final stage just because one of those little fuckers got lucky and blocked your attacks six times in a row.
The weapons are somewhat underpowered, too. Even with everything maxed out, you'll still be struggling in the later stages. Which is quite annoying because improving weapon stats does take a fair bit of grinding. Especially when it comes to weapon damage and reload time, most weapons could use one or two more tiers.
And that's a shame, though, because everything else about the game is great. The art and music are awesome.