Just a theory, but I believe the fight is time-based rather than enemy count based, as you should reach the end of the trip with a small amount of oxygen left.

If you turn around and see a Necromorph that may have been sitting there for a while, just shoot him once and look for others, focusing on him only once you have confirmed that there aren't any others ready to attack. Focus your fire on the last Necromorph, as the others will not attack and will give you time. If there are multiple Necromorphs ready to attack, shoot each Necromorph once with a low damage gun such as the pulse rifle.Shooting a tentacle will cause the Necromorph to give up on it's attack, even if you do not do enough damage to remove a limb.You need to spot the Necromorph the instant he shows himself in a window. I remember having some trouble with the elevator, but here are some tips: If shot: Delay for a few more seconds and retreat.Delaying for a few seconds to give you a chance to attack.Choosing a window and displaying their tentacles (weak point).'Didn't even take half my health in damage, but I can't hit a moving target, so a third 10 minutes lost to one boss battle. The last boss was just my final 'God damn, this fucking game.' You do everything perfectly just to fuck up the quick time event (the very last action of the game), resulting in a restart of the fight. Playing this on easy and still rage quitting when I beat the original trilogy several times. I keep hearing 'the remake is too easy.' I've damn near beat Elden Ring. Just hordes of the exploders in between various necromorphs. So many fights felt frustrating, not scary. They replaced the 'Alien' style of a few strong enemies and made it seem like a 'horror' Call of Duty with the fights. They overcomplicated some of the puzzles instead of making them more intuitive. After playing the entire game, I am VERY, VERY much a fan of the original, but I can't stand the remake.

Dead space 2 final boss easy