I'm on my second run through, this time on casual (first run was on normal).
I installed the Genki spawn mod (sets him to 3% chance spawn on one of the islands).
He doesn't fair so well when hit with the "little gun" from (hidden location).
All in all I think I banked up pretty good, one time I swear he drop'd around 240K. I was just using regular melee instead of shooting him or hitting him with the bat...
Side note: if you buy the Gibbing option for the bat, then don't worry about upping the damage. It pretty much insta-gibs anything...
Now back to Genki hunting