Bad Christmas Tharja

Just use her.
Most of her damage comes from her Special Procs, dont worry about the little cut in Atk.
She basicly plays like any Otter WTharja, wich the HP boon she even get some more universal tanking Power

Any W!Tharja is perfectly usable - she's that good. Her Attack with a bane only goes down to 31 from 34 if I recall correctly which is still reasonable. Higher hp also gives her a larger window to use Vengeful fighter and other hp requirement skills (its quite meager however). If you really don't want to use her though, she also comes with amazing fodder in the form of Candelabra, Close Counter and Vengeful Fighter

As mentioned above, her damage comes from glacies in enemy phase. There are worse banes imo.

Give her a Rauðrblade+ and Hone and Fortify support and everything will die. Mine is -Spd +HP, has Rauðrblade+, Aether and Armor March and she's the one of my key units for Grand Conquest. If you're not keen on investing that much in her keep her Candelabra and give her Glacies (it works better with Vengeful Fighter) and a new C skill and see how you like her.