You can't win with small, balanced groups. You have to zerg the mob with a high number of players. They don't have to zerg lower tier encounters just like the lowest tier guilds doesn't have to zerg orc camp 1 in EC. But, at the top end, it's still zerg tactics.
Source: wiktionary
The scale and lack of focused content doesn't facilitate this grouping as well as some other games. The good side of that is you don't get the "wonderful" experience of zerging around DF with everyone else waiting for something to spawn.
Source: wiktionary
No one ever did D (or just 3-4 people tried), we just zerged, and we would lose every single AV.
Source: wiktionary