Recently I was trying to use this, I spawned Emelita behind Aht and tried to use the attack, however it was grayed out. I equipped Emelita's combo and she was able to use it just fine. The attack says it costs 8SP which Aht has. I just don't understand why I'm unable to use this skill.

If you look up the skill in the game info the skill has a range of 0, thus to use it someone has to attack you, it cannot be used as an attack.

It can be used to attack. Combo skills with a range of 0 you just need to position your ally adjacent to the enemy rather than adjacent to the partner as when you attack an enemy with melee the square used to attack is the square the enemy is on so if you have the the 2nd character (the one that's not using the skill) stand adjacent to the enemy then the character attacking initiate the enemy you can use 0 range combo skills on them.

Yeah, however I don't think it's wise to position Emelita in front of Aht in most situations.

It's a defensive move. Monster attack Emelita, then stand right next to her, Aht jump in and bash it down.