I'm sure I'm not the first one to think about this, but "Greeks! Let's, um, fight stuff!" does not fit Percy as a war cry. You can't tell me that the guy who did a complete harangue out of nowhere when he was almost 16 said that at the beginning of another battle, just no. But I'm still thinking that the sentence itself is good, specially because you can see the difference between greeks and romans.
So, who should've said that? Since there's no other leader figure for the Greeks than Percy (which is a strange part of the plot but that's for another post), I've come to the resolution that it should've been Nico di Angelo the one who said that. First of all, just five minutes ago he had made an entire speech to both Romans and Greeks, so yeah it would've made total sense. Second, he's an Aquarius (I'll never miss an opportunity to remind everyone of this), he would totally say shit like that out of nowhere, and because of this same reason he has a lot of iconic lines, still making a lot of sense. And finally, at that moment Nico was in a rush of adrenaline, I guess. He knew he was nearly dying, Gaia had just woke up and nobody knew what happened to Leo, but he was with Jason and Reyna, so he had enough passion to encourage everyone else, and then he can go stop Octavian with even more passion than before.
Plus, if he led the Greek demigods into battle, that would've helped him in so many ways when trying to integrate in camp.
