What am doing now is getting my 4* Effie to level 40 to use her in the arena now.
The fact is, I am going from my most mobile blue unit: Catria, to the least mobile: Effie.
I'm probably going to have to switch out Seliph for Roy just to Shove Effie.
If Effie isn't worth using to pull off perfect win streaks, I'd rather spend 20k feathers on another blue unit. My only other one is Female Corrin at 5* but, I have a few lower rarity ones I could upgrade to merge to her to raise her stats.
However, that's just an example. I don't think I would use Female Corrin because there's becoming too many wyrmslayers to deal with.
Running 4* Effie (I'm assuming the 4* is not a typo) in the Arena is probably a little tricky. The balance right now in Fire Emblem Heroes is really wack since it basically boils down to who can one shot the other first for most people.
Unless I've screwed up my math, I'm fairly sure an attack neutral 4* Effie with +4 attack from Nino's aura cannot one shot HP & def neutral sword users, except Lyn, during the enemy phase. The +4 attack she gets from the Silver Lance+ upgrade resolves this issue.
The main draws of Effie are that she has the second highest average stat totals, needed to run up high arena scores, and that she doesn't need a lot of set up to one shot every sword user in existence. Without the Silver Lance+ upgrade and a lot of people stockpiling on magic nukes, it might be a little tricky to get through the Arena without any losses if you have to spend extra moves finishing off almost dead sword users.