|As the name suggests, this build will transform Gwendolyn into an extremely bulky enemy phase tank that is still capable of dealing a lot of damage. Being on a full armor team, specifically with Ward Armor stacking, further compounds her defensive capabilities.
IVs: A resistance boon is necessary to further reduce damage taken from mages (as well as healers and dragons). Speed is the bane of choice because it does not take away from her bulk in any way. In fact, she needs to be as slow as possible to maximize her damage.
Weapon: Gwendolyn has base 38 defense and 28 resistance (31 with the resistance boon), so Berkut's Lance+ is taken with the resistance refinement upgrade to boost her initially unimpressive resistance. Of course, her resistance will be further increased when she is attacked.
Assist: The assist skill is a matter of preference: Swap allows Gwendolyn 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: Her special will be Glacies, and while that seems inferior to Ignis, Gwendolyn will actually have more resistance than defense. The resistance refinement increases her resistance to 35, and she gains another 7 when attacked, giving her 42 effective resistance.
A Passive: The A skill is relatively obvious. Distant Counter will allow Gwendolyn to counterattack any unit that attacks her, regardless of range, which is essential on an enemy phase build such as this one.
B Passive: For the B skill, take the newly revealed Vengeful Fighter 3. While this is definitely more expensive than Quick Riposte, it has a lower threshold of 50% compared to 70%, and while above that threshold, the user has a built-in guaranteed Heavy Blade proc. Because of this, Glacies will activate every turn Gwendolyn is attacked as long as she is doubled, which she will almost always be thanks to her very low 21 speed. (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, Gwendolyn should 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.
Sacred Seal: Distant Def 3 is the sacred seal of choice to round out the build. When attacked by a ranged unit (all of which deal magic damage except for archers and daggers), Gwendolyn will gain 6 defense and resistance during combat.
Here are Gwendolyn's stats with this build when she is attacked by a ranged unit.
Alone: 54 HP, 44 Atk, 21 Spd, 44 Def, 48 Res (38 damage Glacies)
With three armored units each with Ward Armor: 54 HP, 44 Atk, 21 Spd, 56 Def, 60 Res (48 damage Glacies)
When all is said and done, Gwendolyn can truly become nearly unbreakable with this build at its maximum potential.