This will come in time if you spend enough time playing Versus.
For the most part, boomers will get you the most amount of damage, especially if you hit 3 or 4 of the survivors with the bile.
A few other tricks:
As a smoker or hunter, stand near the alarmed cars and other like panic events. Most survivors'll see you and automatically fire at you, possibly hitting the vehicle. Quickly dodge behind the car or jump away as a hunter, the horde should swarm them, and then either tongue or pounce them. The attacks of the horde will gain you damage as you hold them down.
Similarly, using teamwork, a player covered in bile makes for an easy grab, and if they're being swarmed by infected, any damage dealt will add to your total. I personally have dealt nearly 300 points worth of damage just from this tactic in a single grab on two seperate occasions.
As always, look for stragglers or people who run ahead to get some damage in before the others can catch up. The infected take no damage from falling, so use that to fall from high heights and catch them from behind for a quick attack.