Just finished this. Strategy I used:
Domina
Definitely a lot harder, and more up to RNG. I ran Demo and used a Voidwalker. Blueberry takes 40% less physical damage, and my Felguard (even with the conduit) was constantly dying.
Build
Ignore stuff related to Felguard because you don’t use him, otherwise pretty standard. Dreamweaver is really good because if you die and go into the pod you drop aggro on the Ambushers. Legendary doesn’t really matter. The Drain Life healing conduit is also particularly good, as you’ll be using Drain Life a fair amount, as is the Demon Skin talent for a huge amount of passive shielding.
Strat
Assassins have something like a 100 yard range and only attack players (Blueberry is safe!), so your goal is to be as far away as possible from them. They spawn in random locations on her platform, so some of this is just getting lucky with spawns and hoping they’re closer to the 4th boss than to where you’re standing.
Place a Demonic Circle on the red X, use pet move command to put blueberry on the blue
X (take a pull or two to find the max distance you can drag Domina out before she resets, this is where Domina will be tanked), and put a gateway along the green arrow. Pet should be passive, and make sure all of Blueberry’s abilities are turned on. His threat generation is automatically turned off when he’s summoned into a heroic instance, so you might have to manually turn it on.
Pull Domina with Curse of Weakness, take the Gateway, run a little up the hill, and Teleport to your circle on the red X. When Domina is about 20 yard from Blueberry, tell him to attack. Once he’s grabbed aggro, put him back on passive. This will trigger him running back to where his move command was to. Once he’s there, tell him to attack Domina. You shouldn’t have threat issues for the entire fight.
Try and get as close to a real opener as possible. Drums, potion, GFG>Vilefiend>Dogs>Tyrant, and so on. Your goal is to kill her before the third set of Assassins spawn, so dps is reasonably important.
Alternate between healing and doing damage. Remember, if Blueberry dies the boss will run to you and almost instantly reset, so you can’t rely on Fel Domming a new one out. That said, because you’re killing her so close to her reset point, you can just reset her if something goes wrong. It’s impossible to be completely out of range of all possible assassin spots, so if you get unlucky with assassin spawns on the first wave, feel free to just reset her. On the pull I killed her I actually didn’t have drums, wildseed, or a potion up. I just got lucky with the first two assassin waves and it didn’t matter.
Margrave
Super easy, 0 rng. Just go Destro with Blueberry. When adds spawn just havoc them and keep dpsing the boss. Don’t bother running into the soak circle they put down. With Demon Skin you literally outheal toxic rain, so the only way to die is to get hit by tentacles or somehow get overwhelmed by adds.
If you’re worried about your dps here, spread out your cooldowns between the phases: e.g. use Infernal during P1, potion during p2, and drums during p3.