We're all ignoring that we recently got a look at him fighting the brute. It wasn't something we could really focus on but we did see the aftermath. Given what we know about the Shishigui, (massive buffers, resistance to Qui drain and use of burst damage attacks and poison), I think we can make a few educated guesses about his build.
"There was a crash, and Zhengui cried out as a blazing meteor struck his side, only to fade and reveal Ji Rong, rising from his knees, bloodied and wounded, cradling his spirit beast in his arms. His foe stood from the crumbling rubble of the kennel, badly burnt but unbowed."
He was able to fight near equally with something of equal stage while it was being mass buffed.
He wasn't able to protect his thunder snake while doing it.
-He was holding the Snek when he was knocked back. Sounds like he caught it, we should target the snake for bonus emotional damage.
He didn't manage to spot the Dancer who attacked us (Neither did we but Six did).
He was angry when we retreated outright, so he wasn't as cornered as he looked (Or was arrogant enough not to see it).
Mostly guesses as I said but I'm thinking he's probably got some way to temporarily give himself a massive buff. I can't see how he could go toe-to-toe with that buffed brute otherwise. That he protected the Snek suggests the buff might be reliant on it being around which would fit with it calling them "blood brothers" or whatever earlier on. That or he's overly attached to protecting it and we can exploit that too. Not being able to spot the dancer says good things about our chances to avoid him but I can't imagine he'd challenge us without some way to find us.
I think most importantly we should remember that he's blunt but cunning, a mob boss really. I doubt he'll use dispels when AoE's and destroying the ground will do the job. Same with protecting himself. It's probably quite easy for people to find out our strongest attack is touch based, so he'll plan for that. If there were bookies on this upcoming fight I'd put money on us going for the finish and finding he has reactive thunder armour or similar. I think we could definitely get too far into it but if we can't get any info on him then basing a strategy off of his personality might be our best bet.
Again, mostly guesswork but its about all we've got so far as I don't think he's fought anyone publicly since coming back with Lilling.