Playing my warrior and rogue in 2s and 3s, shamans are the easiest things to burst down. A 5 or 4 shaman + healer 5v5 works because they have the burst to take someone down from 100 - 0 in 1 salvo and no heal will be able to heal through it. Or at least thats the theory. In 3s and 2s, shamans just do not have enough burst damage by themselves to take someone down quickly enough to finish a game.

Unfortunately 2's and 3's are completely mana fights for the most part and elemental shamans end up losing that. >_<