Shallan’s trait Fae Dust (Your creatures deal 5% more damage with attacks and spells and take 5% less damage (up to 90)% for each time they’ve Attacked or Cast a spell in the current battle) does not increase the damage dealt with attacks or spells. The damage reduction does work as expected.
Also there’s a typo in the “(up to 90)%” where the % sign is outside the parentheses.