A grenade would, admittedly, come in handy at the moment. Even a gun. But Souji has no idea how to use them, never thought to learn from Naoto when he was in Inaba, and he certainly isn't going to do it now. So he marks it at the back of his mind - find some means of acquiring a gun and the supplies to make grenades.

Later, though. Izanagi can't rend the large ayakashi in one strike, even with a pierce attack and Megidolaon, but that's fine. He doesn't worry about it.

Just lets his Persona draw back, in the same moment that he rushes forward. There's no need to relay information to him, nor to Homura, not when the Persona is a part of him, and he trusts Homura to cover him, to understand what he's doing when he brings his sword up the same time Izanagi does his naginata.

They both go to strike it at the same time, to take its head off.