|Reinhardt is a major player phase threat, thanks to a combination of his high movement and his Dire Thunder, which is basically a Brave tome. He's got a good enough Atk stat to put it to good use, letting him dispatch a lot of units he comes across.
Weapon: Dire Thunder is the only weapon Reinhardt needs. It strikes twice consecutively, and those two strikes are usually all Reinhardt needs to shock his foes. It also drops his Spd by -5, but since his Spd is already low, it is rendered a moot point. That is also why a -Spd bane is optimal on him, as it is a dump stat and would not compromise his defenses. However, he'll be fine with any stat variant as long as he's not -Atk.
Assist: Draw Back is a good Assist in general to use for ranged units. Its uses are more defensive, allowing the user and an ally to move back one space. Ranged units usually don't want to be in the line of fire too much, so Draw Back can help them retreat. Reposition is another good Assist, which can either help bring an ally closer to the fray, or bring an ally out of danger. Cavalry units put it to great use with their high Mov.
Special: Moonbow is Reinhardt's Special, which lets him pierce through an enemy's Res. However, he needs to have the Quickened Pulse Seal when using this skill, as he'd be able to activate it on the second strike. If the Seal is unavailable and Reinhardt is using a different Seal, then Luna would be preferred instead. It also lines up well with the rhythm of his attacks. He'll be able to strike twice and defeat an opponent, and his next victim will take two strikes, with the second hit having Luna charged.
A Passive: Death Blow 3 is the only A skill Reinhardt needs, as he's strictly a player phase unit. It grants him +6 Atk when he initiates combat, which can help him power through a lot of units.
B Passive: Lancebreaker 3 is an option for his B skill, which can let him annihilate any lance wielder. He would be able to perform quadruple strikes against them, almost assuredly firing off his Special as well. However, this skill slot is ultimately flexible, open to be anything else the player needs. Some options include Wings of Mercy 3, which could help improve his already great mobility, letting him warp over to an injured ally and potentially catch foes off guard. Chill Res 3 or Chill Spd 3, while far more elusive, could help support his team by debuffing an enemy unit from afar.
C Passive: Goad Cavalry is an option for Reinhardt's C skill if he's on a cavalry team, letting him buff the offenses of his cavalry allies. However, this skill slot is flexible, open to be whatever else the player needs depending on their discretion and team composition.
Sacred Seal: Quickened Pulse is necessary as Reinhardt's Seal if he is using Moonbow, as it helps bring the charge down to 1 so that he can activate it in his first engagement. Should it be in use or is unavailable, then a simple Atk boost with Attack +3 will help maximize his power.