The current version of this game is literally impossible; do not waste your time on it.
The first level grants you a laughably nerfed Gatling gun against two densely packed blocks of infantry. You do not have any further assets to deploy against this attack.
You will not be able to win this fight; the regular weapon cooldown between painfully short bursts will guarantee the enemy advances faster than you can kill them, guaranteeing you lose. While you get a reward based upon the number of kills you make before dying, there is nothing to spend your rewards on.
Again, the current version of this game is literally impossible; do not waste your time on it.