Thought this might be useful for anyone struggling! Plus I'm very proud that I didn't use a guide for this gold trophy :)
I used the Creature Creator to capture Azi Dahaka with a Large Pod in Bevelle. I was going to level him up and release so that he becomes Bahamut for one of the trophies to get 99999 damage or 99 chain, as I was having difficulty with one of them, can't remember which.
When I started levelling him up in Via Infinito I used the Valiant Lustre Garment Grid for Moogle Cureja and Mascot which upped a lot of his stats and Ribbon as an auto-ability. Then I equipped him with Admantite for Auto-Wall and maxed out defenses, then a Speed Bracer for Auto Haste. He was able to beat the Mega Tonberry and most of the 80+ levels of Via Infinito fiends.
So when I got to level 100, I decided to let him have a go at the final boss. He has previously learned Damocles Photon as his own auto ability and learned Flare and Bated Breath from other fiends. I scrolled through all the items and accessories to see what I could feed him to add to his abilities. I gave him 2 pixie dust for the Auto-Life spell. He beat Paragon but I didn't realise you can't heal between Paragon and Trema so his MP was gone. I fixed by equipping Ragnarok for Spellspring and compensated with feeding 2 Speed Bracers for Auto-Haste. I kept Adamantite of course. I didn't realise that pressing R/L3 actually changed the monster's strategy for attack/defense, and I added Darkness as a skill but I couldn't get past Paragon so I unlearned it, as well as Damocles Photon as it wasn't as strong as Flare. Unfortunately he relearned the Photon attack in the battle again which slowed it down a bit. For good measure I gave him 2 Souls of Thasma for the magic booster to max that at 255 as well. It took about an hour and a half but it was beautiful to see it all succeed!
Of course I had to equip Iron Duke after the battle to give him his reward!