Golden Sun

It's possible to beat him with a party in the mid-30s or even lower; if you have double that, you can get by on sheer fortitude. Just spam Wish-type moves, spam the best single-target attacks, and use some stat-uppers.

Put all of your djinn on standby mode then summon all out.
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Definitely need a healer with either Wish or Aura (or something else similar!) to heal the entire party every turn. I'd recommend putting most of your djinn on standby, but not more than you need for a few major summons (the best of whatever you've got). I usually set it up with one always attacking/summoning/using Psynergy, one almost always attacking/summoning/using Psynergy, one healing, and one supporting the healer and summoning/using Psynergy.

I remember making the water elemental the healer, the earth elemental had a revive spell in case she died,another person had 30 revive items (i never ended up using that many ). Used djinn for physical damage instead of weapon unleashes. That way you have 3 attackers no more than 4 turns away from fusion attacks 80% of the time(percentage is probably higher, i'm just being safe). It got the job done for me with all 4 of them at lvls in the 40s.

That is definitely a high enough level; I beat it at level 30 for my entire team,and I saw someone do it at 26.
The character classes you get for having the Djinn stay with the adept of its own element (ex. Isaac has Venus Djinn) are the best for this battle. Every turn, have Mia use Wish (or Wish Well), Ivan use party-boosting psynergy, your Gaia Blade wielder (Isaac or Garet) use physical attacks, and have the other one use his strongest attack psynergy (Heat Wave for Garet, Ragnarok for Isaac). If the dragon kills someone, have Isaac use Revive (if Isaac dies, use Water of Life). With this strategy, you should win without breaking a sweat.

Spam Flash with one of the characters, and you can beat him using any strategy. You can spam Flash and then spam Breeze if you don't want to summons Mercury Djinn while attacking with the other two characters.

Wow, over twice the level you need to be, and you still cannot kill it. That is sad. Anyways, just give Spritz, or whichever djinni heals the entire party to someone besides Mia. Have all Mia's djinn except one be Mercury, and have the non-Mercury one be on standby. Then, spam Flash and Spritz while having Mia attack with water psyenergy or summons.

There is a trick to it i beat the fusion dragon with all level 28 charecters the trick is that the fusion dragon has either 5000 or 7500 health im not sure which but it has high defense to win you need to constantly have mia use her healing moves like wish or ply well on the whole team so that their health is at max each turn and hit them with judgement meteor thor and the water 4 djinn summon. after 2-3 shots with each one the dragon should go down easily it does help to get all of the djinn

Just Use Granite and Corona when you can, set them when you cant use them. Use Spritz a lot too.