it takes few seconds for 15 archers to kill jason. Since its DM there will be more than 15 archers. But with bellephon speed, he can just run around.
Ody is not a good hero at mythic age cause most mythic age myth units like collosus, chimera, fire gaints etc have high pierce armour. Hence you cannot always depend on ody himself. A good hero like bellerophon is always needed at the battle spot. Ofcourse its not easy to kill poseidon's mythic age hero cause he has huge HP.