Our comp had only 1 decurse as well (ele shammy) and it wasn't too big of a problem. As others have said, make sure only 1 shaman is up at a time. they curse 3 people at a time and as long as its decursed every cd, it shouldnt be a problem
We've done it before when our only decurse was our bear tank, so obviously we weren't getting any decurses. Just make sure you only have 1 shaman at a time and nuke it down asap. Other than that just heal through the damage and don't get yourself killed.
There is no problem at all with only 1 decurser, we have a single mage, and curse have never been an issue. In fact ppl exagerate how bad is the curse for ur raid, its bad only for ur tank, since the extra dmg might kill him. We only lost some1 to curse when a 300k chain lightning hitted some1 at the same time, but that was a missed interupt, not curse's fault. And we do have 2 shamans up, since we aoe after the dinomancer, no issue with them, they die as flies and cast curse only once anyway. Killing bear then shaman 1 by 1 felt like there is too much dps loss on boss. Tbh lightning totems in the mess felt more threatening then the curse.
About targeting shamans on top of the bears, are u sure it is not just a normal feature to make the fight easier for ppl struggling on this boss on normal?