It only has to be immune to the gripping effect, since you will be glyphing your death grip to not taunt with Glyph of Tranquil Grip. (Do you want to die to a melee swing?)
I tried to maximize this DG spamming playstyle in raids and dungeons back when I first got my hands on the set bonus in the beta. It's a giant hassle, and a total pain in the ass to attempt to do in my opinion, so I would be immensely dissapointed if it turned out to be viable in raids.
I tried to play with extra death grip bindings to spam it more easily. As in, I bound DG to an extra key on my keyboard, and to a key on my mouse. The mouse keybinding ending up being the most useful, as it removed a lot of the stress from my left hand.
The second method I tried was to have 5 extra macros keybound to spam DG together with my abilites. An example would be
Code:
#showtooltip Frost Strike
/cast [mod:shift] Death Grip
/cast [mod:shift] Frost Strike; Frost Strike // probably don't even need the mod here, but i'll have it here to show my point.
Now copy that for 5 abilities, Oblit, FS, SR, HB and a mouseover HB (seriously, if you play frost you should have a mouseover HB button), and you've just lost 5 semi good keybindings and you're stuck being careful about which version you use, depending on if you're targeting the boss, or another enemy that is immune to the grip effect, or you're targeting a smaller add that is grippable, making you lose out on your 2pc for 25 seconds.
Seriously man, I hope this crap never becomes viable in raiding. all that needs to be done is to make it only proc when used on players or their pets, totems, and so on.