It's a 2 heal fight.
It's a bare minimum to get 4 orbs every time (so you get 5 stacks of the buff).
You should be aiming for 1 add before 1st transition, and 2 before second transition.. it's not such a big deal to have more, but you will run out of raid cooldowns to absorb damage from explosions and your healers will be oom at the end.
The fight should take about 8 minutes max.
This is our alt run of last night on it:
http://www.worldoflogs.com/reports/w...?s=7787&e=8258
As you see, 2 of your healers are doing.. whatever they are doing, but 27k hps? the dps is hard to compare, because obviously, the less orbs you kill, the less dps you'll do.. but should be 4 waves, ignore 5th and focus boss. Everybody should do 100k dps+ on this fight with the damage buff from the bubble.
As I said, check our log, was first kill of a very undergeared alt run.