I'd like to clarify something about the last phase on heroic (10-man), specifically dealing with the last Echo of Attenuation - contrary to what's mentioned in the guide, the Echo was completely immobile in all our attempts, once the boss spawned it (which normally happens after the first Force and Verve in phase 2 and occurs simultaneously with Convert), it stayed there until the end.
This may present an additional challenge if you decide not to kill the Echo: the tank has to make sure s/he is always close enough to it so it doesn't start channeling the Song of the Empress, and that may not be easy if the Force and Verve bubbles spawn in a bad spot or if the Echo spawns in an awkward position (e.g. very close to a wall) that makes avoiding its Attenuation more difficult. Divine Shield and BoP help a lot with this because they can negate both Attenuation and Force and Verve, but you probably need to make sure you burn the boss down before you run out of cooldowns.
On heroic, the shields reduce damage from Sonic Rings (the yellow ones) by 75 %, so you can pretty much ignore those when standing in the bubbles. This may help a bit during the double Attenuations followed by Force and Verve. Unfortunately they don't work against Sonic Pulses (the green ones) so you still have to avoid them.
The Echoes have 1/10 of the boss' health (21.8 M) and killing the last one is a viable strategy as well; it's what we had to do because people kept randomly dying during the double Attenuations.
On our first kill we had 3 minutes left until berserk when we killed the Echo, then two DPS managed to die somehow within the next 30 seconds and a third one when the boss had about 7 M left and we still killed him with about 5 seconds left until berserk, so with everybody alive the 11 min timer should be easy to beat
Also, the order in which the boss uses his abilities can be a bit random and probably depends on how fast you're pushing him out of the first 3 platforms, but in the vast majority of our attempts the last phase went like this: Boss Attenuation > Force and Verve > Convert + Echo Spawn > Boss Attenuation > Echo Attenuation > Force and Verve > Echo Attenuation > Boss Attenuation etc. We had a few attempts where he completely skipped that first Convert (which made killing the Echo significantly easier) or did an Attenuation before spawning the Echo (which was confusing), but we couldn't reproduce any of these situations reliably and most attempts followed the pattern above. It's likely that the boss is a bit bugged.