Astral Chain definitely varies with the level of difficulty for its different bosses. Some require more strategic maneuvers to take down, while some others are for the most part straight forward. Fights like Cerberus need a bit of a plan to avoid taking massive damage, but the followup fight with the chimera Diomedes definitely is a good bit easier.
As we recommend with most fights in Astral chain, start off by binding Diomedes with your Astral Chain to get some good hits on him before he is free. Also take advantage of his stunning to activate IRIS and find out what his health is by moving to cursor on top of him.
After he breaks out of the chains, just keep hitting him while dodging his attacks. None of these seemed to do an incredibly amount of damage and your attacks, whether they are the default or the Gladius X-Baton, seem to deal very quick damage to Diomedes. Like with previous bosses, also mix in some Sword Legion Slashes as well when you get an opportunity, as that can deal damage even faster.
Compared to some of the other bosses that take a little longer to whittle down, Diomedes has pretty low defense and you should be able to take him down in very short fashion.