Could you maybe try a mage heavy strategy? If you can hone Alexander to R3 (or even R2 supplimented by some other AoE magic like Meteor) you can take care of the adds easily and achieve the target score. Then just unload onto Belias with various high level spells. You only really need to worry about magic mitigation in the long run because eventually Belias switches to just using magic once you've damaged him enough, so if you bring Wall as your RW, that should be enough to soften the initial physical attacks from him and the adds near the beginning of the fight while stacking with Shellga and the breaks for magic mitigation as the battle progresses.
In short, have a dedicated healer with Shellga, a support with Magic Breakdown & Full Break, and three mages (A summoner for Alexander and another strong spell, someone with Meteor/AoE and a single target ja spell, and the last mage with two ja spells), I think this is a fairly obtainable strategy if you don't have any other options. If any of these characters has a Hastega SB, even better. (All while bringing Wall as your RW, as mentioned above. Alternatively if you have a native Wall, bring a Hastega RW, preferably one that boosts magic or resistance.)
