Ah, this is a bit challenging. You don't have any dispel scrolls (shops don't always offer them) and many of your skills aren't great for boss fights because they require staggered, just don't work on bosses, or are more geared for AoE fights. You may want to pick up some single target spells for Act 4 and the Attacker trait on your physical damage dealers. At level 15+, it's a nice boost to damage/striking power. Plus, the AoE spells have a much lower chance to inflict debuffs than the single target ones, and Slowed, Shattered, Scared are all huge on this.
I'll be honest, with this party setup, it feels like fighting a boss on nightmare rather than easy.
With your party, I had the best luck with Shea, Krimm, and Rowinda back row, Riley, Jariel, Althea in the front row. (I went with Krimm because you don't have paralysis dispel and she has a huge amount of paralyze resistance so she can break out and keep up the bleed easier.) I didn't actually beat it; I got him to 80 HP but then he cleansed and then he healed up. I could probably do it if I tried for a bit more, but I'm limited on time.
My strategy:
Shea/Rowinda put scrolls of haste on Riley, Jariel, Althea. (Tip: if the character has at least 1 turn left on the 60% haste buff, you can give them a weak potion of haste and it'll give them +3 turns at the 60% level, so you can save your more expensive scrolls until Deceit dispels your party.) If everyone on the front row has at least 4 turns of haste, they switch to using mana/healing potions (even the cheap ones) just for the low delay. If they get unparalyzed and the front row is hasted, then I had Rowinda use Deadly Swamp (praying for Shattered) or Cyclone if Deceit already has shattered. Shea would use Earthquake to inflict Shattered or Fireball if Shattered was up for the extra dot damage. The healing spells aren't needed because you'll have plenty of turns tossing around your million healing potions.
Krimm tosses out minor healing/mana potions until she's unparalyzed (she gets 0-2 turns). I'll use Spinning Strike to inflict bleed if he's not bleeding, normal attack if Deceit is high on HP and/or the turn predictor says she'll go again before he does. Mighty Blow if he's at half health or lower. Normal only hits for 12, Mighty Blow for 40, but the delay on Mighty Blow is kinda nasty, so using it when Deceit is high doesn't seem to help much.
Riley puts up Magic Chains if the boss isn't Slowed, otherwise he does Shadow of Death for Scared.
Jariel puts up Potion of Slowness if nobody else can put up Slowed before Deceit will go, otherwise he just tosses heavy explosive bomb. (Even on easy, he seems to miss 15% of the time. Someone should teach him how to aim.)
Althea is complicated and probably the key. I tried Take Aim + hide in shadows + Critical Hit, which if you can pull it off (and the boss has Scared + Shattered) hits for ~250. But, Take Aim/Hide in Shadows both have a long delay, Cleanse wipes out Take Aim, and Curse of Whatever dispels Hide in Shadows. I managed to get the super combo hit off once, all other times part of my combo was dispelled and by the time I set it up again he had already healed out of the danger zone. I think I had better luck just tossing heavy bombs constantly.
Maybe you can play with it a bit more and figure something else out. Sorry.
Maybe fabula has the key?
