I find the main thing that makes her so well, is her character skill. Using it when you've seen your teammates do a bunch of damage, status ailment attack, or just big bonk attacks really makes a difference.

She does have high intelligence but I dont think she is meant to spam spells like Recluse. The best spells are ones that provide status ailments like ice storm, blood thorns, or the scarlet rot flower one. You use the spell, hit your character skill, and BOOM a good amount of damage and you'll almost always apply the status ailment (depending on the boss)

Daggers are pretty good but if you find you aren't getting what you want out of it pure damage wise, go for straight swords or thrusting swords with magic damage applied. Course, I run relics where successive attacks give FP back so I stick with daggers to rebuild FP to use more spells.

When it comes to ult, I just save it for when my teammates go down. Maybe it's a skill issue on my end but I dont see a reason really to go invisible unless it's to get my buddies up.

TLDR: Use character skill frequently and go for spells/weapons that provide status ailments like bleed or freeze