Just to emphasis something from above:
"Raid" mobs--even those doable by a single group--almost always have higher resists than yard trash mobs or even named mobs. To really do much to them they need to be fully debuffed, which means Malos, Tash, HoS, and probably Malosinia. If you are doing these single-group, and don't have a bard and/or enchanter, you may find it very difficult to land dots on them. And keep in mind that the same debuffs affect DD damage too--so debuffing doesn't just help shaman or necros, but anyone who does spell damage. So debuffing as much as possible is definitely in order.
But shaman still have lots to keep them busy even if they can't dot or DD well (and IMO shaman attempting to dps on most boss mobs are getting very low value for their time and mana). Heals, cures, laying on procs and Champ, etc. If you ever see a shaman doing nothing during a fight with a boss mob, you are probably looking at an afk (or lazy) shaman
Fudapuda
Bertox