Anti-Multiboxing strategies
Though I would tell about some things that are good vs multiboxers.
1. Siege weaponry. Damage is based on how close the projectile hit, a good hit deals ~10% of total hp on EVERY character. A few vehicles and turrets that focus fire can cause some serious damage. Also vehicles have further range than players and move faster than a player can run so they are really strong.
Since turrets and vehicles die within 1-2 hits from a multiboxer with a big team it can be good to have people with strong CC to buy the siege more time to kill them.
2. Abilties like Arcane Torrent, War Stomp, Shockwave, Shadowfury, Grenades are very effective since they cc everyone.
There is several druid teams with 20+ characters. I think they should be more vulnerable to fear than any other multiboxing team. (no tremor totem, no dispel I think. It also makes it harder for them to hit the target with their spells since they ran away and probably face wrong direction)
Their most dangerous ability is Starfall. If they lose control of their character Starfall does no damage.
This means that it is possible to stun, fear (warrior, priest, warlock), incapacite (grenade) and possibly some other things to stop them from dealing so massive damage.
3. Main character also often has a lot more healing on it and is easier to heal than the slaves, so it can be more efficient to kill the slaves.
4. Some weaknesses for a shaman team would be Arcane Torrent (bypasses grounding totem) and stuns that doesn't get stuck in grounding totem.
5. If leader gets CCed the team can't move until CC end or the multiboxer change leader. A good way to prevent them from reaching turrets or vehicles, or just preventing them from getting to key objectives.