Here is some math that will give you an idea of the required DPS.

In 8-player mode, the tanks combine for 2066834 HP. The enrage timer is 390 seconds. Now there is a total of 100 seconds (4x 25 second shield phases) where you aren't damaging the tanks. So the 'effective' enrage is 290 seconds. If we assume that tanks will average out to 800 combined DPS then the average DPS per DPSer is 1581. Your melee classes should be doing much higher than this, and so this brings down the requirement for the ranged.

FWIW, melee shouldn't be doing less than 1700 in this fight, and if your range are sitting in the 1450 range, you should be in a good spot.