WHM99/BLM49 with trusts
Used a bunch of trust combos but nothing worked. Eventually I realized that the Poison effect had a limited range so I used as many long range trusts as I had. I used Gessho for tank, 3 RNG trusts and Ullegore (blm) for additional Stun and decent spell damage. The RNGs typically stayed out of range of the poisonga ability and I used my WHM/BLM to Stun Mellonia as much as possible, typically just the IV/V spells. Doing that seems to prolong Mellonia's use of Ematic Discharge (poisonga) and that is critical because the longer that poison is in effect, the stronger the poison becomes. By the end of the fight, the poison was dealing damage of over 140 hp/tick. So the fewer party members that need to be healed, the better...hence the RNGs. Shantotto would be effective but tends to use his MP too quickly, but he does at least stay out of range.