Wierd that you get that amount of dmg within 10-15 seconds. We are 2 man healing it ever since we killed it first time , both on normal and hard mode.
From hunter perspective:
your hunter should use Cobra Shot before orbs/sparks spawn in order to have his focus bar filled up , and save instant shots / abillities for orbs. I personaly don't use serpent sting on those since they're duying very quick so it's waste of GCD.
Also tell him /them to save some minor dps CD's for later spawns (3rd onwards) and use them wisely , one CD per new wave should do.
We're experiencing problems with those only on hard mode and it's usualy only our ele shaman that can't kill 4th orb , even tho he's one of our best dps'rs in general.
In general your burst classes shouldn't need help for orbs , it's mainly certain casters that should be supported by tanks.
4+4 orbs is perfectly viable for both normal / hard mode if you struggle with those.