For nexus, the dragon is the last boss. But for the mage boss, have mana totems down, then when she does the air toss, drop healing stream totems to cover the dmg, spam /follow macro and just quickly regroup after it.

The void boss is pretty easy, just nuke, switch to the rift, kill rift add then back to boss, no real dmg concerns tbh. For the big dude, real annoying as he hits hard, but i just strafe on the ramp to avoid the crystals and gradually take him down.

Last boss is ugh and took me 6 tries. I spam jump to get rid of the debuff. Bit of dps, then heal tank then heal self, repeat. I spread them in a semi-circle to avoid the aoe breath but still be in range of chain heals. Start off with mana totems and then at 50% switch to healing totems so you can dps a bit harder. Blow all trinket and spam thunderstorm every time for mana. Pot if you have to (i didnt but got very close to needing it). Recomend using fire eles at the start as they can alse take the chains spell :P I forgot to check if grounding totems take it too, would make life a heall of a lot easier.

I've managed to 4 man the nexus (4 ele shammies) and it is a real pain in the ass imo.

Edit: All lvl 70 btw