I never said anything about them not being able to use the Phalanx. I just said they used crossbows.
The Epirus Bow isn't all that OP actually. Thesues sent a few guys flying backward. And Hyperion blew up a stone door with it. It's no more effective than a catapult or ballista. And Thesues isn't immune to arrows or swords. All they have to do is light him up or swarm him.
Achilles will slaughter a good number of guys, but in GoT they don't fight fair. He'll have 30 guys trying to hack him to pieces at once. He's not gonna survive.