Observations (some were already noted before):
-Mega boss uses hate reset every 5% if it has TP, but doesn't store it as a charge. The hate reset timing is animation based. When hate reset occurs, the adds will use their respective SP, and the mobs don't hate reset if they have their SP on.
-When adds are killed, every other remaining mob levels up.
-When BLM is killed, if you do 50k damage to mega boss in a single attack, it levels up.
-Secondary note to BLM being killed, it results in mega boss taking zero damage from skillchain (and magic?), but the damage is still calculated towards the 50k damage. For example, 49k WS -> 49k WS = 0 damage SC will make the mob level up, since the resulting skillchain likely brought total damage to 50k.
-Casts protect and shell, so use dispels/finale.
-All adds resist Drain III, but the mega boss does not.
My group found it easier to just have everyone group up after killing the BLM add and rely on GEO bubbles for defense, keeping stoneskin/blink up on the mages.
Yagrush WHM helps a lot, but is not a requirement. Afflatus Misery + Esuna and remedies will work fine, but everyone needs a decent level of magic evasion gear on D or higher. Jungle Hoodoo can be an issue due to the extreme potency of its paralyze effect.
We also favored PLD over RUN due to RUN relying more on applied buffs... but I think the difference is marginal if the player on RUN is well geared and skilled. Beginner RUNs will struggle a lot, whereas beginner PLDs can relax.
Adds should be left up as they will level up the mega boss making it much more evasive, however the BLM can cast death so it is advisable to either kill, silence, or sleep it. Additionally, if everyone stands close after the BLM is dead, it can be easy for a PLD/BLU to keep hate on the adds even after hate reset.