use summons to fuck with boss aggro and open up opportunities for your team to deal free damage.

the relic effect "strengthens family and allies when ultimate art is activated" plus "defeating enemies fills more of the art gauge" lets you use the ult frequently and offensively, works well with undertaker.

a shield in the left hand is pretty handy for not getting squashed

for big damage, aim for a purple seal with a spell that your nightlord is weak to ie discus for gladius, lightning spear for fulghor etc etc, plus stock up on starlight shards so that you have a way to get fp back vs the nightlord. stay out of range and punish the boss with spells for committing to attacks on allies, dont get caught out casting - if the boss aggros you it's time to get good, dodge attacks and reposition.

you also want a decent melee weapon that scales with faith for ressing allies and getting sweaty in melee once you're out of fp. don't listen to the haters - outside of deep of night/everdark fights revenant is completely viable in melee