I tried this in the T15 tier because I was sure I would zerg him in the last phase. However the tentacles in Phase 2 were really hard to kill since only Unleash Elements (15s CD) would damage them while gripped. Apparently the legendary cloak proc can also damage them so I guess I lucked out big time. Besides holding UE for the tentacle grip I used EM+PE default PVE build for the final phase burst.
The MMO-C Shaman Solo Thread:
Ulduar Interior Yogg-Saron
YGO-GX Supreme King Theme
BABYMETAL - Catch me if you can