One you get to the caves, you'll need to find the Hexagon Crank, this is found on a dead Bravo team member. Take the first door as you climb down into the caves, or head straight to find a save spot and item box.
After you find it, head back and you'll find a gap that can only be passed by using the crank.
You'll come to a small corridor with a boulder at the far side, go examine it then run for your life back to the door you just came through. After you're safe, go back to where the boulder was and keep going to find some ammo.
S/he is susceptible to flame rounds from the grenade launcher, otherwise a few magnum rounds well also down it.
The attack pattern is pretty simple, shoot twice at a medium-close range (or long range if you're using a magnum), then run around it. Be careful though, its lunge can sometimes hit you. Also, watch out for the spider on the ceiling.
You may want to bring a blue herb as well, as the attacks tend to poison if you're hit. First Aid Spray doesn't cure poison.