Practice Segments

Start playing the whole game in one go, sure. However, unless you're actually recording an attempt, save! Otherwise you don't get to the ending often enough. You want to take some time with each segment until you nailed it. Maybe try a new strat. Speed at this point doesn't matter but being comfortable to not get hit.

Save-anywhere and No-Damage mod

A great resource is the F5-save-anywhere mod, e.g. save before a difficult passage. You can also change ..\Directors\Haydee\PawnMenus.dir in Edith > Graphite - there is a branch that tests for debug - just remove that.

Another idea is the Immortality mod - just keep going even after you got hit! Trainer Control https://steamcommunity.com/sharedfiles/filedetails/?id=2500935501 has both, F5 and more max health, and allows you to run faster so practice isn't as boring.

Headshots

Practice headshots, even if it takes longer to aim (for now). Stand still. Notice if the head moves left or right. For slashers you need to memorize where they run to, eventually. Some of them stand still long enough to be stunned by the first shot. Most, however, run towards you not on a straight line. Timing shots right is a matter of practice. And with practice I mean the enjoyable moment once you notice progress!

Aim rather high at the forehead of creepers, for slashers just below the red dot. With the damage upgrade the big guys need exactly 3 headshots, the scissor ladies only 1 headshot and one center-mass.

Important: notice the cross-hair changing on a hit! This tells you exactly wether you missed and have to back-up or can go forward with your second and third shot.

Don't Shoot Nice Guys

Find out which guys you don't have to kill, who are just standing around and are not attacking. There are a lot! You can even outrun slashers twice, on the Security run-around, and on the stairs up from Technical to Medical at the end.