Hey man i'll do my best but its been a while.
First off most of my weapons were tier 2 and i'd say my characters averaged around 4500hp if that's a benchmark I can use? Anyway, cast the 2 useful shrouds before the battle as buffs and debuffs are a MUST.
Im really sorry about how brief this will be but im in work and so Im restricted but start off with Com/Rav/Sab until you've slowed him then unleash rav/rav/com mixed in with some rav/med/com. When he's staggered use com/med/com rather than all out attack and this should allow you to remain well healed rather than risking anything.
Use med/syn/sab ANYTIME he removes his own slow debuffs, anytime period, staggered or not, just do it and when in trouble i went with med/med/sab I think iirc.
He only casts doom when youve taken 20 minutes and done correctly this should take nowhere near!
So, to reflect:
Com/Rav/Sab (to start battle)
You can actually ditch relentless assault from that if you feel diversity is enough its up to you. Hope I could help man!