The Eternal Cylinder

An Omnogrom appears as a large, downward facing mouth with legs, and attacks when it senses you by opening the mouth and attempting to walk over you, then biting down. It's eyes are located on the sides of the mouth, near the bottom, and can be hard to see, especially if one is actively chasing you.

To squirt water, make sure your water sac is highlighted in your inventory hotbar, then hold the right trigger to start shooting. Adjust your aim into the creature's eye and continuously squirt it until it runs away for this achievement.

If you can't manage to sneak up on one, you can also try getting it's attention so that it chases you, then swap to another Trebhum and move off to the side for an easier aim while it targets someone else. If you lose a Trebhum in the process, you can reload a save after.

Edit: There is a variant of the Omnogrom called a Blue Omnogrom, which appears in snowy areas and is identical in appearance save for its thick blue fur. It behaves the same way, and will run away if sprayed in the eye, but I cannot confirm if it works to unlock this achievement, since I had it before encountering them. To be safe, make sure you do this on a regular, naked omnogrom. They are more common anyway.