Resident Evil 0

For the Defeat All Leech Hunter Enemies achievement have Rebecca pick up all 50 of hers, and Billy pick up 40 to spawn the last enemies that are required. This will also allow Billy to either carry the shotgun/GL (if you have ammo left for these) or pistol and ammo for the pistol otherwise you won't be able to reload.

• You have a limited amount of ammo, and there is just enough to Kill All Enemies. It's easy to end up going all out firing everything, especially when Hunters or monkeys are involved, but avoid the temptation as you'll need every bit of ammo available for Defeating All Enemies.
• Your worst enemy is the Monkey/Ape in the right side basement as they are fast, often dodge shots, and can lunge from a distance.
• Always kill zombies with the handgun/knife.
• Try and use Rebecca as an item carrier. Play as Billy and try standing a couple steps in front of Rebecca to limit her chances of being damaged by any area attacks, or in some cases leave her behind until a room has been cleared.
• Give the shotgun/grenade launcher and magnum to Billy, though you may need to give them to Rebecca to switch in the normal pistol and reload.
• Use the inventory to un-equip the AI partner's weapon where it may waste ammo, I'll mark this with a (!) in the guide where it would be good to do so, otherwise have the AI equipped with a pistol when entering the next room.
• Pick up any health items you find, and drop them in the main hall or in the upstairs podium room. Keep at least one health item with you at all times. As you clear areas you'll often be heading through these rooms so you can heal then.


On to the actual guide...
1)
When Leech Hunter starts, check the herbs in front of the stairs in the hall. If it is the Green and Blue herbs and no leech in the middle of the stairs, continue the game. But if you found two Green herbs, press start and quit the game and start Leech Hunter again so you get the ideal item placement for this guide.

2)
Start by taking the ladder in the main hall and head towards the wrecked train, picking up the shotgun and it's ammo in the water. Equip the shotgun and open the door to the train area. Then step forward to change the camera view and start shooting the hunters. Be patient with the shots, spending no more than 2-3 shotgun rounds per hunter so you have at least 11 shotgun ammo left. Rebecca can be assisting with a couple of pistol shots. Pick up the green herb and fire key, then go back to the main hall.

3)
Leave the green herb in the hall, and go east through the door to the dining room. As soon as you open the door kill all the zombies with the pistol. Use the fire key to unlock the kitchen but you don't have to go in to the kitchen until later.

4)
Open the door to the corridor (!). Kill the two zombies on the floor by walking over them with Billy and stomping them when they grab you. Go down the stairs, past the boiler and down to the basement in to the statue puzzle room. Equip the shotgun and run forwards, as you get to the end a Hunter will lunge at you. So, when the camera angle changes step back to avoid the attack and shotgun the hunter to knock it over, run over, aim down and shoot it once more. This is the only way to kill Hunters with two shotgun shots.

5)
Don't go through the other doors yet, instead go through the gate by the statues that is already open and another Hunter will try lunging at you. So quickly step back or be quick at aiming and shoot it mid air. Kill it, then equip pistol and open the door on the right side. Kill the zombies then get all herbs and the Grenade Launcher (GL). If you can't find the Grenade Launcher here press start and quit, otherwise beating the game with A rank is very difficult and the location of weapons will be different. Go to the other door and kill all zombies again with the pistol.

6)
Open the door to the place where Billy saved Rebecca in story mode, killing all zombies with handgun. Then open the door to the stairs, and equip Billy with the Grenade launcher while having Rebecca stay at the stairs. Head down the stairs, killing all spiders with one shot each from the GL. You may need to aim up or down if they are close or on the ceiling. Equip the shotgun again and open the door on the right and kill the bugs by the waterfall.
The room at the end is empty so you don't have to go in there until collecting time. Head back to the main hall.

7)
Leave Rebecca in the hall. Equip the GL with Billy and open the door to the west side. Turn the corner and kill the Mimicry Marcus leech guy (should take three GL shots), open the end door into the projector room, it's usually empty but kill any zombies with pistol if there are any in here. Re equip the GL and go back to the hallway and open the restroom door and kill the other Mimic in here before picking up the shotgun ammo and handgun ammo. Send Rebecca to the projector room to pick up the health items. Return to main hall.

8)
Leave Rebecca in the hall again and go down to the main basement (where the Marcus painting was in the story) with the GL and kill 4/5 more spiders. You have to move quick to avoid their poison attack. Bring Rebecca in, and enter the room on the right side, kill all zombies by handgun, and re-equip the shotgun and open the door to the torturing room. Killing all Monkeys in here is the hardest part. Take your time, then pick up the new shotgun and drop the other if there is only a round or two in it. Leave the room.

9)
Go to the door in the end of the corridor, open it, kill another monkey, open the other door, kill two more Monkeys. Pick up the Magnum, and return to the main hall.

10)
Go to east side, back through the dining room, and in to the next corridor before equipping the Magnum (!) Open the door to the storage room. Kill three Hunters in front of you, a single magnum shot to kill each one. Be patient and they will walk round the corner. Then run to the place you found shotgun in the game avoiding another hunter, and kill it from behind. (If you keep running until the very end, you can avoid Hunter's attack and get behind the Hunter.) Re-equip handgun, go up the ladder, kill all zombies outside.

11)
Unlock the door on the end of the outside corridor, and go through the door into the podium hall. Equip the Magnum (!) open the knights door and kill all Hunters in this corridor. Open the fire key door at the end, kill the three zombies inside on the floor. You can run over to these and stomp them when they grab you to conserve ammo. Go back into the corridor and through the double doors to the library and kill all zombies. Head through into the small side room, kill three more zombies and pick up the handgun ammo.

12)
Return to the podium hall, and take the next door in the south west corner with the Magnum equipped (!) and kill more hunters in this corridor. Open the door to the first aid room, kill zombies and pick up first aid spray. Swap out Billy's pistol for the custom pistol on the bed as it has better critical hit chance. You may want to take Billy's pistol back to the podium room and drop it there if there is still ammo in it. You might need it later for Defeating All Enemies.

13)
Go back to the main hall, and open the door toward the centurion boss area from story mode. Go up to the large open area but stand right at the top of the stairs. Kill the bugs with the shotgun, let them group up and come toward you as they are unable to reach you here. Open the knights door on the north end, and kill more zombies with pistol. You will need to aim down at the ones in the center. Go back down the stairs you killed the bugs from and open the door on the corridor with the shotgun ready, and kill two more bugs. Go toward the blue door at the back, equip GL (!) and head through.

14)
This next area has three Mimics, one in each corridor and one in a side room so use the GL, but you might need to use shotgun and pistol to kill the last one if you weren't careful with ammo earlier. Open a door and kill whatever is in inside, there are zombies in a couple of them, some on the floor, and the very end room of the second corridor has two hunters in it, so try and keep enough shotgun/Magnum for these. Return to main hall once these rooms are clear.

15)
The last two rooms to clear are the upstairs east and west side doors in the main hall. West one has crows, the East one has zombies. Pistol them.

Assuming you have marked up all leeches on your map you can open up the map to count them up to 90 before collecting, or the 100 you need for the A rank. Put the GL/Shotgun in Billy's inventory so he can't collect more than 40. Go back through everywhere sweeping every room for leeches, be thorough as some are hiding a bit, like these;
1, In the small room right next to the library. Upper level, behind the moving bookshelf and on the floor.
2, On the lift in the library.
3, Right next to the one ammo GL in the Hunter Room. (Blue door area)
4, In the kitchen, beside of the refrigerator. (On the counter, both green and blue leeches are hiding. You can barely see they are blinking.)
5, In the outside walkway, two behind the column.
6, Next to the ladder in the main hall.

Once I had 90 I left Rebecca in the main hall and went hunting with Billy.
If you are struggling for ammo remember there is the pistol Rebecca has, the one you dropped earlier for the custom pistol, and also the knife in the dining room if you run out completely. Should also be some herbs left over.

Areas where I found the last enemies I needed to kill, though they may be somewhere different for you;
• One bug enemy in the waterfall room in the basement. Do this first if you have shotgun/GL left.
• Zombies in main basement side room.
• Zombies in the statue puzzle room.
• Zombies on the outside walkway. Go through the store room where hunters were or you'll get grabbed at the podium room entrance.
• Zombies in corridor through the Knight door in the podium room.
• Zombies in corridor through single door in podium room.
• Zombie on floor in room at the end of corridor above. There are three in this room, one is before the movable table, another on the other side kind of under the table, and one by the fireplace. Don't know if I missed this one earlier or if another spawned.
• Last two for me were in the kitchen through the dining room. Had to use a knife for the last one after I ran out of ammo...

Took me about 1 hr 40 minutes.

This is a MUCH MUCH easier way to get this achievement as well as the A ranking. The sub-machine gun method is extremely easy and didn't even break a sweat. Had plenty of ammo left and plenty of healing items. I take no credit for the video, I'm just linking it to help others:

If you want to do this the easiest way possible I recommend beating the game on easy while going for the following achievements:






Once you beat the game on easy difficulty you can then play Leech Hunter mode by loading the clear save and selecting extra game. Whether you use the machine gun method or magnum method you will kill all enemies with plenty of ammo left over. I recommend you use Billy only until you have eliminated all enemies besides the respond enemies which come once you have collected 90 total leeches. It is recommended you collect 40 with Billy then collect all 50 with Rebecca. I left Rebecca by herself again while I eliminated the respawned enemies then I collected the last 10 leeches. The only enemies that spawn after collecting 90 leeches are zombies and 1 bug.

Easy mode gives you the same amount of ammo but easy mode makes it possible to kill enemies much quicker with the below required shots:

Bugs take 3 handgun shots. (For the bugs in the big centurion room, you can shoot them with the handgun at the top of the stairs and they can't attack you.)
Hunters take 1-2 shotgun shots or 1 magnum shot.
Zombies take 3-4 handgun shots.
Monkeys take 2-3 shotgun shots.
Mimicry Marcus take 1 grenade shot when they have transformed.

You can unlock the following three achievements playing Leech Hunter mode on easy.






*** It is important to note that you must beat leech hunter mode on at least normal by collecting at least 90 leeches to unlock a weapon for the following achievement but thats easy when you can ignore half the enemies doing so. ***

I'll second that the machine gun is the way to go. The GL will either have flame or acid rounds and the spare GL will have 2 frag rounds (the magnum method gives 30 frag rounds and 1 in the spare which is so much more effective against the damn apes and hunters and clones). The machine gun goes so much further than the magnum with extra ammo.
On the magnum strategy, i couldn't even kill them all or get 100 leeches. This version left me with a few grenade rounds and about 60 left over pistol ammo when all was said and done