But if you use trainee with onslaught, then does she still get a power boost when attacking a monster since they will be banished instead of just destroyed?
That is what I thought. I took her out of my deck awhile back because I thought I saw her do nothing (and it doesn't look like anyone else uses her, either). I didn't know about heirloom, though. I'm guessing she doesn't work with that because it destroys by an effect. In a way, it is kind of a shame. She could have been good.