When you fight the Leech Zombie, its legs will swell up at the end and explode, hurting and often killing you or your partner. There are three ways to avoid this. First, just run through a door as his legs are swelling but before he explodes. Make sure that nothing is attacking you or your partner when doing this. Return to the room and there will be no extra leeches. The second is to use a Molotov cocktail on him, which will light him on fire and melt into a puddle of slime and leeches. The third is to kill him while he is still in "human" form, which is best done with the magnum and has the end effect as the Molotov cocktail.