Gessho helps you during this fight. He is not an enemy. The Ashu Talif Captain, if killed immediately, wins you this fight I believe. The crewmembers dont need to be engaged if not necessary.
I should also confirm that you only need to get the Captain down to 20% HP to complete the mission. Our set up was:
WHM BLM NIN THF DRK
Each of them soloed a mob in the first wave, with the exception of the NIN tank, who took on two, then, everyone focused on the Captain in the second wave. BLM/WHM is very handy in this fight, as I had to use Benediction, which instantly got the attention of ALL the mobs, and Curaga from the BLM took the heat off me a little until the tank could get the hate back.
Did this fight a week ago. It's crazy. The whole party wiped toward the end, due to vicious WS damage from the Captain, and with just me(75 WHM)and the Captain left standing, I only just beat it down enough with my capped club to get the cutscene for us. I was left with 37/1038 HP and 17/1033 MP. There were only 5 of us (mostly 75 but one or two a couple of levels lower) as the 6th (2nd BLM) couldn't get into the BC arena. Close fight, but wouldn't be too hard with a full party of good players.
Ghesso is indeed an NPC helper, and he has quite a bit of HP, so no need to concentrate on him. I think he can be cured, etc, but I didn't bother. Too much to focus on with party members.
Luthien is correct. The Captain is the formor dressed in the COR AF.
Gessho is a member of the Tengu branch of the Yagudo, who left Mindaritan long ago before the main branch of the Yagudo degraded into semi-mindless savages.
According to some Japanese mythology, the Tengu were goblins that lived in the mountains of Japan and orginally taught ninjitsu to the Japanese.
"We don't just borrow words; on occasion, English has pursued other languages down alleyways to beat them unconscious and rifle their pockets for new vocabulary." — James D. Nicoll