That's party dependable. If you are playing it for second time (after clearing the game and acquiring weapon components) it is possible to wipe him out at BR 50. If you are playing it for the first time then the best strategy is to upgrade equipment as much (to get Defense as high as possible) as you can and use more unit-specific (not union-specific) combat (Requiem, Blood Rage, Quadrile etc.) and mystic arts (Spark, Wind Shear etc.) with high tiers in anti-mystic formations (such as mystic henge - it's given after beating Jana Royals). Try looking in Mystic Defense Focus Formations on this Wiki. In my case combat arts were quite successful. Mystic arts, since they are oriented on wiping out large quantity of enemies at a time, were mostly useless.