Resident Evil 0

As Billy, route all enemies from every room.

Collect all 50 leeches of one color only.

Exit through the door where you originally spawn.

If you know what you are doing you can use Billy to escort Rebecca through all the rooms. Clear enemies with Billy and collect leech charms with Rebecca. This will cut your time in half.

You must walk through the front entrance and complete Leech Hunter before the achievement will unlock. So don't plan to complete this and the Distracted from the Hunt (Defeat all enemies in Leech Hunter) achievement simultaneously. 90 leech charms must be collected before the extra enemies spawn.

I found a method that I think is both easier and faster than simply taking the 'grab all leeches' path, by taking advantage of the fact there are several rooms you can completely ignore if you know ahead of time which leeches to go for. To that end, here is the path I took. I also recommend using the following map: https://gamefaqs.gamespot.com/ps4/164548-resident-evil-0-hd-..., originally by Mr_Big_Boss just to keep track of which leeches you've gotten so far.

Some other things to keep in mind: You'll mostly be playing as Billy, and you'll be grabbing leeches as Rebecca. Take advantage of Billy's much larger health pool and be a tank. You'll be keeping Rebecca with you most of the time, but only for a little extra damage and to save time grabbing leeches. I will specifically mention when you should split up with and regroup with Rebecca, so if I don't explicitly say to split up, keep her with you. Her extra pistol damage doesn't sound like much, but it really helps when it comes to keeping the hoards of zombies off you. The one exception is if she's in orange or lower health and you don't have a healing item for her, in which case get her safe and also heal her if you can find something.

I'll point out healing items in this guide, but not when to use them - be smart and watch your health. Try to keep at least one healing item with you at all times, but there will be times when you need to ditch health items for the sake of speed (and pick them up later, of course). Make sure you go into these sections with a good amount of health; the best way to do this is by combining green herbs as needed to keep characters (especially Rebecca!!) at full health when you're about to leave your healing items behind. However, most of the time you can just heal by doing the following: when you're getting hit, especially while visibly injured, check your health frequently. The second it turns red, use a full heal.

Don't pick up leeches whenever you see them; they take up your inventory space, so it's best to grab them systematically. I'll point out when I grabbed the leeches.

In general, each monster will be killed using the following weapon:
Zombie - Pistol
Roach - Grenade Launcher
Spider - Grenade Launcher
Hunter - Shotgun/Magnum/Grenade Launcher
Monkey - Shotgun, but not fatally.

Be smart with your ammo. You get more than enough to do this cleanly, but you still don't want to waste it and end up trying to kill hunters with a pistol.

Now, onto the actual approach:

1) When you start up leech hunter, reload until you get the version of the challenge that matches the above map; I found this map setup to be by far the easiest. You'll know you have the right setup when the herbs at the base of the stairs in the entrance room are 1 blue, 1 green, and there is no leech on the stairway landing. NB: If you don't follow this step, the rest of the guide doesn't work because leech placement is different.

2) Group up. Go down the ladder to the sewer tunnel. Playing as Billy, grab the shotgun and shells, equip the shotgun, go through the door. There are several hunters in this room; stay calm and kill them using the shotgun, with Rebecca's pistol providing support. Try not to get hit at all, but it's okay if you do. Once the hunters are dead, switch to Rebecca and grab the herb, leech, and fire key. Return to the main hall.

3) Leave behind both first aid sprays in the main hall. At this point, Rebecca should be green health and Billy yellow or better. Switch to Billy, equip the pistol, and enter the east door to the dining area. Inside, kill the zombies with your pistols. Switch to Rebecca and use the fire key to open the kitchen door, but don't bother entering yet. Ditch the fire key - you won't need it any more for this run, since the other fire door only has blue leeches. Switch to Billy and exit the kitchen through the back.

4) Walk over the two corpses in the hall to trigger them attacking you and stomp their heads. Head down into the basement, through the boiler room, and into the room with the torch statue puzzle. Equip the shotgun. Depending on Billy's health, there are multiple choices here. If it's yellow or better, kill both hunters in the room first. If it's red, go heal (but it really shouldn't be red). If it's orange, do the following: walk over to the doorway that the puzzle opened in the main game. Enter the doorway, and the second the camera changes, aim and fire your shotgun at the hunter. Shoot it again while it's on the ground to kill it. (The other hunter in this room is by the corner leading further into the basement. For that hunter, you want to walk past the corner so that the hunter would see you, then immediately walk back so that its jump misses. Shoot it twice with your shotgun.)

5) We're going to start clearing out the side rooms, starting from the area unlocked by the statue puzzle. Switch to your pistol and start by entering the side room. Kill the zombies in here, then grab and combine the red and green herb as Rebecca, and grab the grenade launcher (it should have 30 flame rounds in it) as Billy. (You shouldn't have any extra shotgun ammo at this point, giving you exactly enough inventory space for the launcher. If you somehow still have some, give it to Rebecca for now, dropping it in the hallway outside this room.) Leave this room and enter the door at the end of the hall (as Billy). Kill the zombies inside with your pistol, switch to Rebecca, grab the leeches and the green herbs. Mix the green herbs together. Exit the room, grab the green herb in the hall, and mix it with the other two. (Pick up the shotgun shells if you left them here before.)

6) Switch to Billy. If you didn't kill the other hunter in this room in step 4, do it now. Once it's dead, enter the room on the right, where you saved Rebecca during the main campaign, with your pistol equipped. Kill the zombies in here, switch to Rebecca, grab the leeches, switch to Billy. Exit this room.

7) Equip the grenade launcher (flame rounds). Leave Rebecca behind for a moment. Go through the last door. Kill the spiders in the hall using the grenade launcher, enter the side door, kill the roaches, and then bring Rebecca over here and regroup. Grab the green leeches as Rebecca and the green herb if you need it, then return to the main hall by retracing your steps through the kitchen. On the way, grab the green leech from the boiler room. Rebecca should now have 8 leeches.

8) In the main hall, play as Billy. Leave behind your pistol and handgun ammo and any extra shotgun ammo you have. You should still have a couple shells (~4) left in your shotgun, but if not, that's okay - you've got a grenade launcher for emergencies. Bottom line, you need the grenade launcher, the shotgun (if you have enough bullets), and 2 free inventory spaces, so heal up if you're hurting. Leave Rebecca here, equip the grenade launcher, and go down the staircase behind the portrait. Kill the spiders in the hall and go to the end of the hallway. Switch to your shotgun.

9) This part is a little tricky. As soon as you enter the room at the end of the hall, aim down and shoot the two monkeys (one bullet). Run past them to the other door and go through. Shoot the two monkeys in this room (one bullet), run to Billy's right, and grab the magnum & magnum ammo. Shotgun the monkeys again to knock them back, run out the door, shotgun these monkeys to knock them back, then run out the far door back into the basement hallway. Now go back to the main hall.

10) Go upstairs into the lecture/podium room and drop off the shotgun and grenade launcher. Back in the main hall, grab Billy's pistol and ammo (and a healing item if you want, although you shouldn't really take any damage here), and have Rebecca follow you again. Back down the basement stairs, this time equip your pistol and enter the side door. Kill the zombies in here, split up with Rebecca, equip your magnum, and enter the other door.

11) Fours hunter in this room, so 4 magnum shots to clear it. Once they're dead, bring in Rebecca and grab the leeches and herb if you need it. Switch to Billy, reload the magnum, grab the shotgun shells and shotgun, and leave. In the basement hallway, have Rebecca grab the other 2 green leeches, bringing her total up to 12, and return to the main hallway.

12) For this next part, I had the following setup: Billy - pistol and ammo, magnum, healing item (G+G+G herb), grenade launcher (20 flame rounds left). Rebecca - pistol and ammo, 12 leeches. I also had 2 first aid sprays left on the ground and both characters at full health, to give you an idea of my remaining resources. Go into the east room and grab the green leeches in the dining room, then enter the kitchen and grab the green leeches there. Leech count: 18. Go out the back of the dining room.

13) Switch to Billy, equip the magnum, and leave Rebecca in the hall as you enter the other door off this hallway. There are four hunters in here; kill em. Pay extra attention to the one that doesn't attack you until you round the far corner of this room. Switch to Rebecca, grab the hallway leech, then enter the room Billy just cleared and grab the leeches in there. There's also a green herb you can grab with Rebecca's one remaining inventory slot, if you're so inclined. Switch to Billy, equip your pistol, regroup, and go up the ladder. Note: Don't forget to switch back to your pistol. Like an idiot I forgot, and shot a zombie with a magnum round.

14) Kill the zombies on the balcony, have Rebecca grab the leeches (total: 24), and unlock & enter the door back to the podium room. Simple.

15) In the podium room, switch to Billy and equip the magnum. Enter the knight door. Kill the three hunters (watch out for the third one around the corner) and grab the two leeches as Rebecca. If you need it, the room at the end of this hallway has a first aid spray guarded by a couple zombies, but if you don't need it then just head back to the podium room.

16) At this point I was out of magnum rounds (see my note at the end of step 14), so I dropped the magnum. Honestly, it didn't really matter in the end. As Billy, equip the magnum and enter the southwest door of the podium room. Kill the hunter as soon as you enter, then switch to your grenade launcher and wait for the other two to step around the corner. As soon as you've got a clear shot, blow them away. Switch to your pistol, then enter the double doors partway down the hall. Kill the zombies in there, then enter the library and kill those zombies too. Grab the 3 leeches in the library (one's hidden behind the movable bookshelf that's too heavy for Rebecca), the three in the room off the hallway, and the four in the hallway itself. That's enough leeches to need another inventory space, so either use or drop the green herb you grabbed downstairs in the ladder room. Current leech count: 36.

17) Return to the lecture hall and grab the two green leeches here, then go back to the entrance hall. As Billy, equip your pistol and enter the southeast room of the second floor. Kill the zombies inside, grab the leech, and return to the entrance hall.

18) Time to gear up. There are two Mimicry Marcus (creepy leech people) in the next area, which means we need our flame grenades. At this point you should have plenty left, but if you have less than 5, then take the shotgun and ammo with you as Billy. (Each Marcus takes 2 flame grenades to kill on normal mode, but you might miss or accidentally hit it while it's invulnerable and transforming.) You'll want to bring a healing item just in case, so between that, the grenade launcher, and the shotgun & ammo you'll need to come back for your pistol later. Take the northeast door on the entrance hall second floor into the hallway with stairs leading to the third floor. Another great thing about this run is that all of the third floor leeches are blue, which means we get to completely ignore that floor! Instead, go into the other room off this hallway and kill the roaches inside. Use your grenade launcher if you have ammo to spare, or the shotgun if you brought it.

19) Split up with Rebecca for now. Equip your grenade launcher and go through the water door. Kill the Mimicry Marcus in this hallway, then enter the far door and kill the Mimicry Marcus in that hallway. At this point all that's left are some zombies and some bats, so use whatever weapons you have available to you to get the job done. Go back and regroup with Rebecca, then return to the piano room. The zombies in here are pretending to sleep, so you can headstomp them if you can spare the health, or just shoot them. Or blow them up. Or stab them. It doesn't really matter anymore. Return to the hall where you killed the second Mimicry Marcus, go through the EAST door (NOT the WEST door, that room has hunters in it.) Kill these zombies too.

20) Home stretch! Pick up the leeches in the room you just killed the zombies. At this point, Rebecca is down to just one inventory slot for her pistol, but that's okay - the only thing left to kill are some bats. Billy can reload her gun if necessary by exchanging items, but if Rebecca reloads before grabbing leech number 41, she'll be fine. Return to the piano room and grab the leeches there, then grab the leeches from the room with the water door, and finally enter the west door on the second floor of the entrance hall. Kill the bats and grab the leeches, bringing your total to 48.

21) Finally, return to the entrance hall. There's one leech on the second floor set of scales and one on a chair on the first floor. Grab them both to bring your total to 50, and exit through the front door to unlock the achievement.