If it's the one I think it is then here's a tip.
DO NOT READ IF YOU DON'T WANT HELP WITH A BOSS.
Use Lightning/Fang/Hope and toss a librascope, it gets all info on all monsters.
1. Nuke down the 4 caster things with a mixture of Commander/Sentinel/Healer and Ravager/Sentinel/Ravager, start with the first to get the increase break time then swap and rip on them with whatever spell they are weak against, whenever you get low, switch to the first and heal up, using the commander to keep the break gauge up.
2. When they are dead, break the boss and summon something, don't expect to break him again, you won't, or at least I never managed it, his special charge ability wipes Break.
3. When boss uses his charged up ability, switch to Commander/Commander/Ravager and just rip on him until he groans, that means his ability will do less damage.
4. For the most part use Commander/Commander/Healer and Commander/Sentinel/Healer if you get into trouble, your commanders will do much more physical damage to the main guy than your ravagers will do magical.
Make sure you have aegisol up at the start at least, if not make sure you have a synergist put shell on everyone for the first part.
The fight took me 16 minutes in total.