Instructions
Run away from the Slobber, and watch his attack pattern. If he chases after you, you've run too far. Let him catch up, but watch out for his suction attack.
Run off to any side, when he starts inhaling. He will almost always shoot thinner at you before he starts inhaling, so watch for this signal.
Shoot paint or thinner down the Slobber's throat. Fire a steady stream of paint or thinner until he starts coughing. Repeat this pattern four times to kill the Slobber.