I imagine a lot of you have heard of this grunt, probably seen the video clips. Well if you are having trouble finding it, it's really simple.
In the driving part of the Maw, get to the point where Fohammer is suppose to pick you up. She obviously dies attempting to do so. So you must continue on. At this time, a new nav point will appear. Follow the hallway towards the navpoint until it gets to between 914 - 918. There on your right will be an opening. Get out of your warthog and walk into it. You'll be in a hallway. Walk down this hallway and take your first right and walk all the way to the end and there will be the grunt in all it's thirsty glory.