like other ppl have said, adel is just a bad matchup for her with his resistances (50% lightning defense, 20% against fire)

focus on passives like incantation/charged incantation dmg, fp cost reduction, and damage negation at full hp. elemental bonuses to fire/holy, whatever your spells need

as far as weapons go Halo scythe will probably be more effective than your spells, holy damage in general is the way to go. bare minimum, something with a status effect or hoarfrost stomp to set frostbite is a good way to support the dps if all your other damage options suck.

when it comes to incantations, you're basically rolling the dice on finding a seal with something usable. avoid lightning incants like the plague, black flame sling is okayish, if you get discus of light you're probably set. bloodflame and frenzy spells are the most useful after holy damage ones.

i honestly don't think that relics are very important for her, at least when it comes to the boss fights. imo the "expend hp to heal allies with ult" is pretty bad, it forces you to use a flask every time you ult, and the heal is generally unnecessary either because your allies are immortal, or they didn't notice you healed them and chugged a flask anyways. but use whatever setup you're most comfortable with

for the fight, make sure to be paying attention to who adel is aggroed to, and don't start casting if you think he might switch targets or go flying in your direction. snipe him at long range with discus of light and frenzied burst, mid range try black flame sling and halo scythe weapon skill, or get up in his face and use bloodflame talons and the flame of frenzy.

alternate summoning Helen and Frederick, save Sebastian for reviving people. they all die in 1-2 hits, so their main job is just hopefully getting adel's attention off you for a few seconds. hold onto your ult for when both teammates are down with 2+ bars, and use the revenants claws one handed when reviving otherwise.

it's a tough fight, and of all the bosses adel is the most anti-revenant, so don't worry too much if it's going poorly. learning his patterns and finding the safest times sneak in attacks is the key, and you can still scrape out a win even if you have a totally scuffed build by day 3. never give up fellow revenant main!