If opp cost doesn't matter she should be in D.

If it does she should be in E.

Based on the rules units with higher exp priorities shouldn't matter since we have to assume Delthea is being used to her fullest extent. The game offers several rings to help her growtb. We have to assume she is taking advantage of those.

At her peak she can be a player phase nuke every turn. Since we have to assume she's being used a healer can be dedicated to heal her. It's not ideal. But it's better than Valbar and the rest of E.

Again, if opp cost was a thing there wouldn't be amazed argument. But it seems like people are allowed to be selective in who they choose to adhere to this rule for.