Hey fellas, I'm just going to mention something interesting I found recently.
Reinhard is vulnerable to "self-inflicted injury" - which includes injuries caused by members of his own legion attacking him. Even the bayonet of a basic legion soldier, not even one of the LDO, was able to cut Reinhard
And this is separate from Reinhard's physical durability. It is explicity mentioned that Reinhard's indestructible body has 4 weaknesses.
1) have the strength to pierce his armour of a legion of souls -> that is, have more AP than his dura
2) Find a ***** in the armour - referring to attacking an already existing wound, OR in Valeria's case, attacking when he uses the Longinus (the later is not applicable here)
3) Machina's fists, or a similar attack, which bring an end to all things regardless of their power.
4) Self inflicted injury
Methods 1 and 4 and mentioned separately from each other, and the bayonett is from a soldier that
Kei was able to kill, so it is clear that Reinhard's "self inflicted injury"
does not require the "self" to have the AP to harm him.
With this in mind, Altair's ability to create perfect copies of her opponents and overtake existences just became a lot more helpful. All she has to do is copy his Einherjar and have them attack him, or overtake their existence, and now she can harm him even without the required AP