The general idea to killing him with 5 amber drakes is:

1. Keep him frozen
- He is immune after 3 times (diminishing returns) where u have to wait till banish phase
- If he spawns adds during his short 'not-frozen' time before the first banish phase, it's prety much a wipe, because they'll kill 1/2 of your dragons
2. rotate with the beam / dps
- To get him down fast you need to get charges on him asap. To do this you start the fight channeling with 2 alts, dpsing wigh 3. When the 2 channelers have ~5-10 stacks you start channeling the 3 others, then unleash the first 2
---> your other 3 will have 10 stacks each now, channel first 2 again, unleash 3. And just keep repeating this.

The tactic is prety simple, but what makes it hard is that it has to be 'perfect'. E.G. I wiped a few times pressing my time stop roundrobin on a global cooldown, forcing you to press it again, and of course your rotation gets messed up