In my opinion it's not classes that are relevant, but rather Marks, Glyphs, and Spheres. Like, Dawnguard is the best item in the game and it's not even close.
Once you get to 999 attack (which is easy to do with the above) then the only thing that really matters is Guardian's Shield Bash ability which works on every boss in the game. So as long as you have enough agility to act first, you can stunlock bosses to death.
Also, the MC is way better than all of the other characters. You can solo the whole game with them on Impossible difficulty. So, to answer your question, here's the setup I used to kill all of the Abyss bosses:
Mc, Guardian. I'm level 38 so I have basically all the perks, including the very important ones Speedster (50% chance to act twice) and Sword Master (ability to equip two weapons) because the extra mark and glyph slots on weapons are significantly more of a boon than any shield. You're also going to want to pickup Rage (25% crit chance) and Explosive, although you don't really need it it's just to kill things with high HP faster.
Gear is:
Weapon 1: Aetherius (Mark of the Hero, Rage Glyph, Power Orb, Power Orb)
Weapon 2: Exotic Rapier (Mark of the Shadow, Rage Glyph, Power Orb, Power Orb)
Bodygear Oukurqux (Precision Sphere, Fighter Orb, Fighter Orb)
Accessory: Dawnguard (Rage Glyph, Power Glyph, Power Glyph, Power Glyph, Power Glyph)
Headgear: Obsidian Helmet (Precision Sphere, Stone Orb, Stone Orb)
Stats: 4,842 HP, 999 Physical Attack Power, 271 Agility, 196 Physical Defense, 102 Magic Defense, 287 Perception.
Attributes: 195% Hit Rate (I still miss sometimes, so not really sure what the deal with this is), 100% Critical Hit Rate.
The Exotic Rapier has the inherent ability "Grants two extra attacks. 15% chance to set the enemy on fire." The Mark of the Shadow also grants an extra attack. This means that for each enemy I attack, it will hit 4 times at once. Each of those 4 hits has a 100% crit rate and generally deals between 5k and 6k damage, for a total of around 20k damage and a pretty good chance to set the enemy on fire as well as cause the Explosion DoT.
The Mark of the Hero turns your basic attack into a line attack, which generally means you're hitting every enemy on the screen for that much damage.
And if you think that's absurd, we haven't factored in the Speedster perk yet, which gives a 50% chance to act again. The Oukurqux body armor does the same thing, but it doesn't stack how you'd think. It's actually a 50% chance to get a second action, then after that a 50% chance to get a third action.
Because of this if I'm lucky, I can do 60,000+ damage to every enemy with just the MC while giving them a couple of significant DoTs. The Aetherius also regens like 800 something health every round while I draw 500% of the aggro.
This is where Guardian comes into play. You can use your first action to stun an enemy (sometimes multiple) and then use your next actions to attack, which will easily restore your TP to full. You'd have to get exceptionally unlucky to run out of TP this way. Also, sometimes the enemy gets paralyzed by the MC but I'm not sure what's doing that exactly, so even more lol.
As for the last boss, I think I killed it in one or two rounds by just dealing absolutely massive AoE damage while keeping two of them stunned. If you're having trouble, the Succubus is pretty helpful because the sleep on Twisted Image seems to work pretty well on bosses.
