OK here is a guide
M1(left mouse)- shoot, upgrade at shop for fire rate and reload rate, after 10-20 bullets, you will need to M2(right click) to reload.
M2- reload and also aoe attack that will set off mines. After you kill enemy, it will leave a corpse, a mine, that will detonate and kill things in small radius. You can set off chain of those mines.
Main point of the game is that you fire your bullets at the girl. which will lower her HP bar that is visible. She has several of those bars and when you destroy each of them, it will start new phase, with different attacks, and more spawned enemies.
Space bar - after you upgrade at shop, you will gain ability to shoot large projectile every few seconds. Buy this ASAP, projectile destroys all in a line it travels, and does shitton of damage to girl HP .