H!Jakob Build Help

Weapon: Brave Bow+
Special: Ignis
A slot: Death Blow 3

B slot: Bold Fighter 3 (Wary fighter 3 if you're on a budget, but he uses bold fighter better than anyone else in the game)

C slot: Depends on your team comp. Ward Armor is always good for an armor team, but you can give him drive speed 2 from tailtiu if you want to use him solo. Really it's flexible and more based on your teams needs.

Seal Slot: Armored Boots. It just became refinable with sacred coins, so buy it when you have the coin and put it on Jakob for that sweet +1 movement.

It really depends on your playstyle, what units you have to fodder, and what other units your going to pair him up with.

If you don't have an armor marcher, it's going to be tough to build a player-phase build around him.
=> Make a quick-riposte slaying bow/monstrous bow with fury. (If you're willing to sack Takumi, close counter is great)

If you do have an armor marcher, Brave Bow + Deathblow and his wary armor will make things very difficult for enemies. (Honestly, this works even if you don't have an armor marcher, it's just clunky. Also, building an enemy phase unit can get expensive and requires a fairly solid team comp.)

If you're not sure about a c skill, ward armor is great on all armor units. He can also run ploys well.

H!Jakob will have a tiny bit of trouble against a fully decked out and powered up Reinhardt. However, he handles brave lyn if she has a def bane.
(probably the two most common units in arena pre 20)

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