Okay, so I know that there are other guides for this achievement and it's been way over ten years, but I don't care. I had some trouble with this achievement and found what appears to be the optimal strat to pull it off, so I've decided to write my own guide in case someone else is going through Splatterhouse right now. So this is how I did it. I'll post each level below and their time limit, along with my personal record and strat for the level.
West Mansion - 12:00 (PAR: ~9min)
As soon as this level starts, grab a blunt weapon and approach the area between the stairs. Some monsters will spawn there. Hold RT+X and smash them away. You should insta-kill them. In fact, you should insta-kill them with RT+X even if you're unarmed, but I'd go for the weapon just in case. The next waves will begin and you should be able to dispatch the bigger zombies easily with RT+X. If everything goes according to plan, you should beat waves 1-3 within the first minute of gameplay. From now on, you should be able to beat around two waves per minute. When large groups of monsters start spawning, try to pool them together and use LT+X to cut them all down at once. After wave 5 or so, the game will start spawning the whip guys and the hammer guys. If there are too many of them, transform and take them out quickly, or line them up and make good use of LT+Y. Avoid splatterkills in general, because they're too time consuming to use every time, but don't be afraid of using it if you need to refill your necro, as it should give you at least one bar back once you do it, sometimes even more. Be careful with the timer on this level as well. In my experience, it didn't stop counting the time until the results screen appeared, which is some bullshit, as that alone took like 20 seconds to happen, between messages telling me I had completed the level and all, so you really better be careful with your timing.
Slaughter House - 16:00 (PAR: ~14min)
Same strat as last level. This one has butchers instead of zombies, which should give you plenty of bladed weapons. Save those for when you need to eat a chunk of health off of some big monster, as cutting butchers sometimes puts them in that stupid long death animation state, which can and will lengthen wave durations. This level will also spawn your first Aegis enemies, which you should instantly kill with LT+Y (try to line up other enemies as well once the Aegis goes between them for extra damage), as well as climbers, which are a pain in the ass to deal with.
West Observation - 23:00 (PAR: ~19min)
This level is pretty much like the last one as well. It should give you a chainsaw, which you can save for the final wave, or to deal with fire zombies, which are your new threat in this level. It'll also spawn a lot of climbers, which are a pain in the fucking ass to deal with. My strat is to hold RT to run towards them and press X right before Rick stops running. There's a sweet spot between dodges where you can hit them with almost 100% accuracy once you figure it out. It's after their first dodge but before the next, but you really gotta nail down the right timing with your strikes. Also have a blunt weapon when you do this, not only for the extra reach but also because it should kill them off in one hit. Be careful with the Aegis enemies in this level as well. Again: LT+Y on them as soon as they appear and you should be golden.
Death Coaster - 25:00 (PAR: ~20min)
This is the first hard level, in my opinion. Here you'll have to deal with fire-breathing clowns, lots of Jens/blue guys, and less weapons than earlier waves. In this level, it pays to do the secret missions. Tear off arms and use them on grabs to kill a few monsters, step on boreworms and try to avoid getting hit to spawn extra weapons for you. Don't worry about completing this on your first try either. Repeat the first few waves a couple of times if you need it so you can nail down monster spawns and position yourself better in the arena. Then, keep an eye on the timer. You should be able to complete about one wave per minute, spamming RT+X on most enemies, LT+Y on bigger monsters and using B+B to produce weapons when you run out. The last wave will spawn a bunch of butt monkeys, hammers and whip guys, so be mindful of necro management. Transform, kill off butt monkeys and leave one of the whip guys dying so you can transform back, splatterkill them and transform again to continue on. This is also the first level where loading takes some more time than it should. The timer doesn't pause while the game is loading between waves, so press Start once you see the loading icon, let it revolve a few times then unpause to save a few seconds.
Shadow of the Wickerman - 26:00 (PAR: ~20min)
This is the famed hardest level. After reading about it here and in other sites, I was worried that I'd spend an entire day grinding out this level, but it turns out that Death Coaster actually managed to kill me more than this. In fact, I beat this level on my first try. Strategy is the same as last level: pause when the game is loading to save a few seconds, prioritize the Aegis, dodge blue guys and transform whenever the level gets populated by big monsters, being careful to leave at least one monster dying so you can splatterkill and refill your necro bars. Also, do secret missions to get weapons and if you're lucky and get a shotgun or a chainsaw, save it for the last wave. The last wave alone took me about two minutes of doing the aforementioned process of transforming, killing butt monkeys, splatterkilling a whip guy, transforming again, and so on. Be extra careful with blue guys and fire zombies on this level as well, and avoid running out of Necro between waves in case you need it for the Aegis, and you should be fine.
West Manor Chapel - 20:00 (PAR: ~18min)
Just like the final level of this game feels unfinished, so does this challenge. It's a tight arena, with badly optimized assets that will tank the performance of the game once enemies start spawning in. This level was my first S-rank, before I figured out all the strategies of the earlier levels, and it was also on my first try, so I believe this one is unbalanced towards being too easy. In fact, I'm pretty sure it can be beaten in under 15 minutes if you do all of the above and manage your time effectively. The best tip I can give you here is to try to stay on the corridor instead of the altar area, as the performance there is a little better and there is less space for the monsters to maneuver around you.
As you can see, the game gives you about five minutes of extra time for each level, so this achievement isn't as hard as it looks. The problem is that the game doesn't teach you to kill fast, then asks you to do it in these challenges, so you have to kind of relearn how to play it. It's all a matter of managing your blunt weapons, bladed weapons and the necro bar, avoiding unnecessary splatterkills, prioritizing certain monsters and having a little luck with the weapon spawns.