Diomedes will appear in Astral Chain File 3, right after you take him out of the Astral Plane and save the civilians. He's armed with a huge sword that can be very difficult to dodge, so in this fight you'll want to keep your distance.
Astral Chains: How to beat Diomedes
Diomedes is armed with a massive sword which he swings in huge bows, making him difficult to dodge. He's also prone to doing multiple spinning attacks, moving around the small arena like a whirlpool. The best way to fight this boss is to keep your distance. You will want to keep your Legion at the top of the Astral Chain, letting it act on Diomedes while you are in a safe place.
The best thing you can do is try to operate from the center of the small arena you are fighting in. Diomedes can go fast and cover a lot of ground when doing his spinning attacks, so make sure you don't get stuck in a corner or he'll easily kill you.
Use your weapon's pistol mode to hit it with combos and sync attacks. Diomedes will do a top-down attack with his sword, causing a massive energy explosion. Dodge away, the dodge rolls quickly towards him. You should be able to get two to three slash attacks with your Legion before you have to go back.
You want to repeat this tactic over and over again, but be very careful. If you are having more serious difficulties, Diomedes will hit you each time for about a third of your health. Therefore, to be caught in one of its combos means death. You will also have to constantly move your Legion around the combat zone, because if you leave it too close to Diomedes for too long, he will simply be knocked out and knocked back, reducing the damage you take.
The best thing you can do with Diomedes is to just play it safe, operate from a distance, and slowly bring it down until you beat it. And don't forget to go for one last shot at the end to get the most genetic code out of this pesky boss.
