| The objective of this build is simple: turn Tharja into a monstrous enemy phase unit with superior bulk, especially in terms of resistance to combat the ever increasing presence of green mages.
IVs: Take a boon in resistance to take even less magic damage, and a bane in speed as this is the one stat Tharja does not need. Her speed needs to be as low as possible to maximize the potential of her B skill.
Weapon:  Rauðrowl+ is arguably the best enemy phase tome because Tharja's allies can be positioned to give her up to 6 additional attack, defense, speed, and resistance during combat. While the owl tome is an excellent choice, it is by no means easy to come by, and as such, her default weapon Candelabra+ will make for a fine alternative, granting her +2 to all her stats during combat when attacked. Regardless of the weapon, take the resistance refinement for increased bulk.
Assist: The assist skill is a matter of preference: Swap allows Tharja and her allies to all stay within 2 spaces of each other (for Ward Armor stacking) and is generally easier to use, while Pivot allows the player to extend the movement range of her by up to 2, as she can move 1 space and then pivot over an ally.
Special: Tharja's defense becomes her offense through the use of Glacies. While she already has a base resistance of 39 (thanks to the boon), this number has the potential to be much higher.
A passive: It's recommended to keep her default skill Close Counter. This allows Tharja to counterattack melee units, which synergizes with the rest of the build thanks to the owl tome. On top of that, unlike most mages, Tharja has the defense for this to work.
B passive: Stick to the default skill Vengeful Fighter 3 here as well. This skill heavily outclasses Quick Riposte through its lower threshold and built-in guaranteed Heavy Blade. As long as Tharja is at 50% or more HP, she will double any enemy when attacked, and each of her attacks will accelerate her special charge. This is where her abysmal speed works to her advantage, as just about every unit will double her. As a result, she will always trigger Glacies whenever she is attacked. (Being attacked reduces the charge to 3, counterattacking with Vengeful Fighter reduces the charge to 1, getting hit by the opponent's follow-up attack reduces the charge to zero, and she can then strike back with a devastating Glacies proc.)
C passive: To maximize her allies' bulk, take Ward Armor as the C skill. Ideally, Tharja can be run alongside three other armored allies that also have Ward Armor, and with everyone staying close together, each armored unit gains a greatly appreciated 12 defense and resistance during combat. Also keep in mind that each adjacent ally Tharja has further increases her stats by 2 during combat for a maximum of 18 defense and resistance.
Sacred Seal: Continuing towards the goal of mitigating as much magic damage taken as possible, Distant Def 3 is highly recommended for yet another 6 defense and resistance when Tharja is attacked by a ranged unit. Keep in mind that with the exception of archers and daggers, most ranged units tend to deal magic damage.
Here are Tharja's stats with this build (owl tome) when she is attacked by a ranged unit.
Alone: 45 HP, 44 Atk, 14 Spd, 40 Def, 48 Res (38 damage Glacies)
With three armored units each with Ward Armor: 45 HP, 44 Atk, 14 Spd, 52 Def, 60 Res (48 damage Glacies)
With three adjacent armored units each with Ward Armor (owl tome buffs): 45 HP, 50 Atk, 20 Spd, 58 Def, 66 Res (52 damage Glacies)
Naturally, it will sometimes be difficult (perhaps even unfavorable) to give Tharja three adjacent allies. This was shown just to demonstrate how much potential this build has. | 
Optimal
All-purpose
Very high investment
Armored
Alternative
High investment
Creative
Arena