The basic idea is to get that bloodsucker close to the cave entrance, where you've got better lighting and room to move around. Bring your toz-34 shotgun since it hits hard up close, and listen carefully for splashing or footsteps when it goes invisible. Stay near one of the walls, so it can't get behind you, and maybe toss a grenade deeper into the cave to soften it up first. The trick is watching for when it becomes visible again, that's your chance to land some good headshots. If things get rough, don't hesitate to step outside for a breather and heal up. One interesting thing is that shooting while it's cloaked just wastes ammo, so be patient and wait for those moments when you can actually see it coming at you. The cave works in your favor actually, since it can't move around too freely in there, just make sure to keep track of where it is by sound when you can't see it.