not even a problem worth mentioning and no, it shouldnt be a simple case of H/T/D/D. You know the mechanic, so take a group which will allow you to beat it. if you didn't and its your first run through, i feel for you but unlucky
Bring the player not the class, to use a WoW phrase. As long as you aren't running all dps, everyone should be able to tackle a boss in a non-infuriating way.
There is a simple solution to this mechanic. When Malgus has about 10% health left have the tank walk down the steps the middle of the catwalk walkways near the ledges. Then, you kill Malgus with all your dps and healers situated behind Malgus. And then when Malgus starts his phase, you have 1 person with a knockback stand beside Malgus and use it.
It only takes 1 knockback to kill Malgus if he's standing near a ledge to begin with. Its really not a big deal. You just have to adapt your tactics to match the resources you have available to you, and not the resources you wish you had available to you.
No, the only thing you can do while he's casting is interrupt via a knockback. THEN
a short time after one person does a knockback (you only have one per person) another person has to do a knockback and THAT
knockback actually works.
Also you will have to kite him over to the walkway beforehand, as he will literally stand in one place even if you're across the fusk!ng room, so if you didn't do that, or have a person with a pull or stagger knockbacks, he is immovable and invulnerable.