Well, he was eventually killed by a staged teleportation "accident" designed to keep him from completing the second phase of a 'port, basically distinigrating him. That initially failed because Synnar has total control of his energy, reintegrated himself by sheer will. So basically, he was dying the entire time he was fighting the JLA, and still tanked their best shots and manhandled them like they were children.
Blunt force doesn't really phase him, and energy attacks are useless, because he can tap into and control energy (And in fact has an alternate energy form, but prefers fighting as a solid being.)
In the end, it took the teleportation tampering, a lot of fighting, and a device designed to disrupt his energy form to kill him, and he still had the will power to assume his physical form long enough to tell Chief Max "It's only just begun".
If it helps, this is a Jim Starlin monster. They all kind of run into each other.
