So as build crafting is really my hobby, I thought I should take a stab at what can counter that zergball.
I was thinking, mobile single target damage?
I mean, the zergball really benefits from the AE cap, so it doesn't make sense to keep hitting the brick wall with our heads if it hasn't worked yet. But what if that tightly packed group of 20 had to fight a spread group of 20 people all using high single target burst builds? The focus on player based AOE would only hurt them in that fight as they could only kill a few targets at a time, also the zerg ball more often then not tends to "train" in one direction. So perhaps just rolling through the zerg will sort of de agro them from you.
So here are the hard facts I considered, Most zerg balls:
- Most players wear light armor
- They have one or more dedicated healers
- They usually shield stack with harness
- They have tons of player based AOE spells
- They have tons of zone control skills (negate, veil, talons)
- They have a team leader
- The good ones are filled with experienced players that don't die easily
Now how can this be countered by single target damage? Well to start the 20 man single target crew would need to have good players in it as well (I'm talking no PUG groups in this fight). A bow would be a weapon of choice, it provides good damage against cloth due to being a physical weapon, offers healing reduction, can do damage at a distance from the zerg, and most importantly offer good mobility. The single target group would also need to spread out and in a way "circle" around the zerg. This would help prevent the group being susceptible to all the AOE pressure the zerg can put down. The players would need to be very mobile, and have good knowledge of where to go when the zerg decides you are their target. Weaving with other players, direction changes and rolling into the zerg and out the back side would be effective measures. The group would need to pick its targets, mainly the support/healers of the zerg ball and try to snipe them out as best as they could. Good use of the surrounding terrain could spread the zerg ball apart. Finally, each player of the single target group would also need to be a survival artist (using what their class offers them) in order to prevent easy kills for the zerg.
One huge downside I could see to this is the zergball making good use of terrain as to not allow themselves to be surrounded, such as sitting in a broken wall. But that is where I believe the zergball should be most effective anyway

And the fun part is, they could run stamina builds and be very effective in PvP!!!
Just some food for thought, as I like to say: If rock isn't working, try scissors.