Finally got Chemtrail to spawn yesterday, but as I feared he kicked my ass and I'll have to fuse a new party before trying again.
You know what the worst, most dangerous thing about him is? It's not the lack of weaknesses, the constant debuffing, or even the six(!) press turns, but his AI is so smart. He'll never, ever use an attack that will be nulled, repelled, or drained. I had a Vishnu that repelled gun and drained phys, and Chemtrail didn't touch him until everyone else was dead and then did his almighty spam.
I think the best strategy would be to have a whole party that resists both gun and phys, so his damage is diffused as much as possible across the whole team.