They were born directly from the "Primordials" so their power must be at least on-par with the Olympian gods. How could Kratos defeat them simply with his Blades of Chaos? It took the whole power of Pandora's Box just so he could best Ares in GoW 1.
"I'll make my own FFVIII CGI movie, with blackjack and hookers!"
Well he also had those relics, which he did use agenst them in the final battle! Also, the blades of chaos are infused with the powers of 4 of the Olympic gods, i'm pretty sure thats all the power he could ask for to take them down!