Who to Promote

Well, if you're just wanting Olivia as a dancer, then she could do that at 4* but it wouldn't hurt to promote her.

Of course, the same could be said about Tharja. Bladetome units don't necessarily have to be at 5* provided their speed stat is high enough.

I have a 4* +10 Olivia, and while she might not be the best in terms of arena scoring (not really caring about that to be honest), she does get the work done whenever I need her to deal some damage (I gave her Wo Dao+).