Here some helps :
1) No need to rush. Take your time. Going faster is less important than avoiding being hit, so go slowly and analyse, step by step. Getting hit reduce points so it's better to be careful than go quickly.
2) Prefers close attack for "the big monster who walks" : hit quickly 2 times then go on the other side (if you was on his left, go on his right he won't change direction while preparing to hit you so he gonna miss) and repeat. It gonna help you having more bullets.
3) the monster who shoot at you, looking like a skull with a snake body : shoot at him 1 or 2 times (more if other enemies around), then wait for his shoot do dodge it, then finish him (shoot or close attack, as you prefer). IMPORTANT : the time between each shoot is always the same. So if you don't know if he has shoot yet, wait for it to dodge it, then you can kill it.
4) the little shooting station/robot (or idk what is it), shooting always in the same direction : you have to bend down to dodge it. Since it's not a big issue alone, start by dodging/ignoring other monsters to eleminate thoses first if they're annoying you. Close combat.
5) The flying one who goes into you : go a little back on where you was safe then face him, and when he's in range for close combat, hit 3 times quickly. If you prefer to secure not being hit, you can shoot.
6) If multiples ennemis : The "big monster" is slow and predictable. You can manipulate him to make him start an attack in an area you'll left to kill other monsters. The "skull" who shoots at you is probably the biggest issue so eliminate him first. If there is the flying one going for you, you can go back to let him goes for you to isolate him from other monsters. Shoots are important to help you eleminate them while avoiding some hits.
7) Enjoy :)