I was able to beat him just now. I used Charlie, Ruby, and Alister.

I used Ruby's potion to force everyone to move tot he top right corner first. Then used Alisters gun on Dark Charlie to keep him moving and unable to attack. In the mean time, Kiba killed Dark Ruby along with some shots from Charlie. Ruby was attacked once and went invis so I used her ability to lower Kiba's tension. After the three shadows are killed you can use Charlies main attack to do 60 damage a pop, make sure to time it so that Kiba is dark and he goes before charlie but after the boss so he can remove the dark weapon buff.

I tried the Kiba + Charlie + Ruby + Alister strategy above, and maybe b/c I was on Hard, I could not get it to work out. Tried many times.

I then swapped Mondoe for Charlie, and w/in a few tries, got that to work. Most of the damage done to the Terror Weaver was from passive poison, along w/ 1 exploded poison puddle and an Acid Dart. Once Ruby was invisible, most of her actions were either delay the enemy or reducing Kiba's tension. For Alister, I needed the buff that maxed his Avoidance tokens at 4, and replenished that when needed. No one died (well, in my 3rd try w/ this team), but I didn't get the Achievement I should have. Oh well.