I have played Yzma and Kronk together for more than 200 pvp battles and I have never seen Yzma's Pasive trigger nor have I seen Kronk Assist her.
This is suppose to be 20% chance of assist and 25% for her passive since Kronk is a Kingdom Character.
Is this a know bug? or working as intended? if so I might just have to give up on my favorite duo