90 in one go on the portrait ghosts huh? That sounds like it could be rough for a few of them. That one in the ice room, the weight training, and the ballroom dancers come to mind. I can't remember names.
And yeah, that describes me playing Fire Emblem. Well, it used to. I've been playing them so much that I am good at them. I still reset occasionally, but nowadays if it's a unit that I don't really actually intend to use or care about... I pretty much don't reset. Unless they're needed for someone else's recruitment or a good item. Getting into the Youtube community surrounding them helped. The uhh, Ironman runs they call them. You forcibly do not reset no matter who dies, and see if you can complete the game. If your Lord dies, then you start over from the very beginning.
I've never done a true one, because there are some units I just like to much, but my last playthrough of Path of Radiance didn't actually have anyone major die ever, so it kind of counts even though I wasn't really sticking to the self-imposed rules.