Full Support Healer for Auto-Leveling

I think of you are auto leveling for a range unit(or any unit in general) then Azama with his high def for a healer is the best healer to be used as a wall, while Wrys is the best wall for Magic attacks, yes they have low attack but they are jsut there to take hits and heal and thye won't die easily to their respective bulk instead of killing(and since they have low attack even if they attac they won't deal that much damage, unless you give them pain so it will be easier for your training unit to kill enemies)
if you don't want to use them then any of the cavalry healers(since you have elise and Veronica go with them.

Better training healers? Bro, if it's defenses that's all you want, think it's time to bust out the Azama, as he has the best overall bulk out of the healers by quite a bit actually

Elise is not a good choice for this purpose. Extremely squishy. Veronica is...very double-edged. Squishy like Elise, but her debuffs and windfire balm have the potential to help your trainee a lot.

My favorite pairing to use in training new units (I don't use autobattle, but I just have to park them in specific spots and let the enemy come to me, so it's still pretty efficient) is Azama and PA!Shigure.

Azama is built (5 star) +Def -Atk, with Pain+++, Martyr+ and Fortress Defense. He walls off a section of the map and gives healing when needed. He takes no damage from melee units on the training maps, little damage from magic users, and he can soften up the enemy when it's needed.

PA!Shigure (I know you asked for healers, but this is tangentially related, I hope you don't mind) runs a Blarblade+ set (anything, he never attacks anyways) along with Geyser Dance, Spur Def/Res and Spur Def. He dances the trainee, then they get 11 extra defense and 7 extra resistance, which means that they will stay alive for a long time, or at least until you can heal them up.

This set-up leaves a third unit for either a Physic+ healer if you need more sustain, or an extra dancer if you want it done quicker, or a hero with a Valor skill equipped. That's my set-up, it works really well for me, I hope it helps you.