CKPQ Bossing Strategy

- Whatever the difference between our strategies, the 3 Grandmasters will be pinned in 2 corners, Red Nirg + Hsalf in one, and Rellik in another.

Yes. Since there are three grandmasters, there are three combinations (permutations):
1) Hsalf, Rellik + Red Nirg
2) Rellik, Red Nirg + Hsalf
3) Red Neg, Hsalf + Rellik

It depends on what you would rather kill first.

Isolating Rellik seems to be the best way to go. Hsalf alone does minimal damage, and all ranged will be able to participate with no trouble at all. Putting Hsalf and Red Nirg together is just asking for both to become buffed, which is trouble for your ranged.

Thus, depending on which boss you would like to kill first, you can:
1) Kill Red Nirg + Hsalf together
2) Kill Hsalf first, keeping Red Nirg with Rellik until Hsalf is downed.

Killing Red Nirg alone is just also just asking for trouble for your ranged.

A pinner would have equal difficulty pinning either combo, Red Nirg + Hsalf or Rellik, but multiple-target skills have twice as much efficiency when used on the combo "Red Nirg + Hsalf", correct?

A pinner would have difficulty, but not two pinners. Also, pinning Rellik isn't difficult. Just rush to one side, use stance, and stand against the wall. Rellik can't stun you. Hsalf and Red Nirg is a huge mistake as Hsalf jumps quite high and disappears, it may be difficult to pin him with Red Nirg.

Unless you plan to fight the Grandmasters one by one, there's little/no chance to avoid Red Nirg's mass att buff.
Another scenario would be isolating Red Nirg from Hsalf + Rellik, which, in my opinion, isn't very wise. o_o

Yes, I agree that's not very wise. Divide and conquer still looks like the best strategy unless all your ranged somehow all die and there's no Rez.