Suggestions for a 4* Tharja +Res -HP

Just giving her desperation and a move skill is all you really need on her. I recomend making her a 5* if u dont already have a magic nuke.
With an hp sacred seal she has JUST the right bulk to live most counters to get her in desperation range as well as use her special effectively.
I used to have a -def +hp tharja and she was my reliable nuke. Now i have a +spd one but i dont see much difference exept for GHB where the stats are inflated.

If you lack for a powerful mage, she can work even with middling IVs. The blade tome should take care of your damage with the right buffs. Maybe keep Darting Blow on her since speed is such a crucial stat for her ability to counter the top sword units in addition to greens. Either that or fury to boot up desperation.

Thorja is a nuke. -hp +anything is perfectly fine to work with.