Either this Scratcher respawns an infinite amount of times or its extremely high chance, but I found this last night while testing the rifle on Scratchers and noticed this guy seems to just respawn A LOT.

He IS dying because I did a few test runs and the kills matched up on the Results Screen, so its probably a good idea to just get most of your kills right here, even for fighting kills with the katana, its like a step forward, slash twice and then run back and repeat but watch out for the other one.

The only issue here is actually your rifle ammo, on the current route we get 3 boxes before this enemy (2 in jewellery store, 1 in 1F sewer + 6 rounds in the rifle to start with, that's 114 rounds total), so now we might want to go into the police station after twin feeler to grab that extra box, making it 150. In addition, NOT using the Hyper Blaster besides the Cafe Airscreamer to save those No Aiming Shots, and use the shotgun on Floatstinger (meaning shotgun for ALL bosses) and save those rifle rounds specifically for shooting kills in the sewer. So basically, don't use the rifle UNTIL the sewer, kill enemies like normal until you get to this guy, then try to have 80-90 shooting kills before the end of the sewer + switch to katana and finish getting your fighting kills. If this really is as consistent as it seems, I wouldn't even bother killing ANYTHING in the school besides Split Head, maybe a few creepers with HB since you won't be switching to the handgun at all. Basically, Any% Hard +150 items until you get to the hospital where you'll begin killing stuff.

Also the distance on this enemy is Middle Range, handgun seems to miss a lot so don't bother trying that.

UPDATE: Further testing has shown this is a thing with ALL Scratchers, not just this specific room/Scratcher. This is still the best/fastest Scratcher to use however (there is actually only one other that is close to being just as fast, which is the one right before you leave the sewer, it is a little further away making fighting kills on it slower, but its distance isn't enough to make it a long range kill either, still, if you somehow have the first one despawn its a good backup).

Getting fighting kills can be difficult, the safest thing to do is to kill the other Scratchers, wait a few seconds so that the game actually unloads them, then you can focus on the specific one you want, otherwise its a risk each time trying to slash twice with the katana and quickturning before the other one is hitting you.