You actually fight a weaker version of Arkanis Gath at the docks for Bodhi's first quest. He is tough with 3 attacks per round thanks to 5 short sword proficiency points but you could still improve him as you have done with his chapter 6 counterpart.
To get a better overview of the situation, I have recently started a new game with an evil party and joined Bodhi. Today I faced Arkanis Gath as a part of the first task which Bodhi gave me. It turns out that this 'weakened' version of Arkanis is almost on pair with my improved CH6 version. He is defined as a Fighter 7 / Thief 21 (Dual Class), and has somewhat better stats than my version. He does have the correct Thac0, HP and saving throws for those levels but unfortunately, he was gifted with the wrong proficiencies

He had five points in Small Sword which is a leftover proficiency from BG1 and no longer used in BG2 I believe. I replaced this with 5 points in Short Sword, 3 points in Scimitar and three points in Two Weapon Style. He will now dual wield a Short Sword +2 and a Wakizashi +1. I've also replaced the Splint Mail which he was wearing with a Studded Leather Armor +2 so he now actually looks like a Shadow Thief. B) Arkanis was then given 5 Potions of Invisibility, one Oil of Speed and 5 Potions of Extra Healing. I'm still working on the proper AI but in general he should first use the Oil of Speed than backstab and heal when necessary using the other potions.
BTW, I'll probably improve the AI for Mook and the rest of the Thieves that fight alongside her, and I might check out the Aran / Thief Guild battle in a couple of days. As for Gaelan, I'm considering making him escape his home to fight alongside Aran in the culmination of the Guild Battle.