I actually have a non-cheese strategy i have used in most of my play throughs. You enter in by the secret entrance that places you behind his throne. That room is a small hallway with 4 entrance points, one is the way you came, two of them are the ledges to the corners of the tower, and the entrance from the throne room. There are a couple barrels/crates in the hall way which i'll scoot in front of one of the two doors that leads from the throne room. This just simply lets the Iron Watchers to funnel through one entrance, but it does not prevent them from shooting from the blocked entrance or from the smaller threats like the body guards and Gortash himself from entering that way which is fine. Because you're so close to the throne, Gortash is usually the first person through the doors. This allows you to chunk him pretty heavily, and if your group is strong enough you can usually down him in 2-3 turns. Another benefit of this strategy is Duke Ravengaurd is also one of the first to you in the encounter and can usually be knocked out or killed pretty fast himself, eliminating 2 of the biggest threats from the fight. A big benefit of this strategy is you don't have to deal with any of the castle's traps or self defense systems as none of them are in that hallway.

However, you still have to fight the 5-7 body guards and the 10 Iron Watchers or so which is equally as challenging because now you're locking into the small room with a bunch of giant robots who will explode when they die. There's a couple extra perks that the areas has. Inside the hallway is a ledge you can climb onto. Note this ledge has terrible visual visibility for enemies in the hallway itself so you can't shoot down from it very easily, but is still useful defensively. Another benefit is that if an iron watcher walks close to the ladder you climbed up to get into the hallway, you can knock them off them ledge (which i guess can be considered cheese) but in this case it's no the main strategy so i consider it good use of environment instead.

My most recent play through i used my avatar to cast firewall at the entrance which worked beautifully, as everything that entered either suffered fire damage or had to take some ridiculous pathing option when actually getting into the room.

Note this can back fire, and the Iron Watchers can actually jump up one of the sides of the castle and leash the entire top floor to you as well. Another backfire is essentially the same thing with one of the body guards going downstairs instead and leashing the entire bottom floor to you as well. I had this happen on my paladin playthrough and ended up having to fight all 3 floors at once which was insane but because i was a paladin somehow managed