>>528825965
How her passive works is not that you get 10% of your AP as extra damage on crit, its that crits do 10% extra damage as magic damage and the multiplier increases per by 10% of your AP.
If you have 500 AP then your crits deal 60% extra damage (as magic damage). They dont do 60 extra damage.

So example if you have a full AD crit build and are doing 700 damage per crit attack you deal 70 extra magic damage in addition.
If you buy Shadowflame for 110 AP this would increase to 147 magic damage because your crits would be boosted by 21%. If the enemy is low this extra 147 magic damage would also get 20% damage amp and be increased to 176 damage.
In addition to this she gets 10% flat damage from her Q passive on-hit.

Personally I'm not convinced this is that good because dealing two different damage types isn't that effective as you can't fit 2 different penetration types into your build. Like you could just buy BT for 80 AD which multiplied by IE is effectively 172 damage. Her AD scalings on her other abilities are much better than her AP scalings too.