Think PLD/RDM would be better... All the seeds Attacks (Including his Normal hits) Are AoE Magical Damage.
This mob takes massively gimped damage from Nukes, So the BLM Should be focusing on Stun and Teir III'ing the adds (One Tier III should kill adds if your BLM isnt resisted)
SAM and BLU should be ranged attacking the crystal at all times (Cannonball for Blue and Etc). You can have the DD Safely run in and do ONE WEAPONSKILL after the mob uses a TP Move or while its casting an Aga-3.
Other than that looks good, i did mine with PLD,WHM,COR<BLM,SAM,SMN and we managed to win with a SAM and his 180 Archery skill, he TP'd on Arrows and Gekko'd the Crystal while it was casting a spell and ran back.
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