Quick question about my role in a DD first room run

[Ghishlain]

As a WHM, you should be aware of how the tank is pulling the pack and if they're tagging everything or not. If they are tagging everything and getting somewhat low on HP during the path to the tank spot, Sprint ahead of him and drop a Cure on him as he's going by so he can resume without too much impediment. The big thing you want to do is kinda stay on the heels of the Tank, but far enough back so you don't accidentally sight aggro any of the mobs s/he is trying to pull.

Once the pack is in the designated position, you'll want to Holy at least 3 times to maximize the stun effect. I'd recommend putting Eye for an Eye up on the tank at this juncture (if you have it) as well as a Divine Seal + Regen.

If you have Swiftcast up, use a Swiftcast Holy followed by popping Pressence of Mind when your GCD is about 50% cooled off then Holy twice more.
If you don't have Swiftcast up, use Holy then one random GCD (I usually use Blizzard II at this juncture) then Holy twice more.

Add more Holy as your comfort level / MP would permit (I generally Holy until about I get to about half MP, depending on group composition). Use Shroud of Saints to regenerate MP at around the 70% MP mark and try to reapply Regen before Divine Seal wears off. Weave Cleric Stance toggles and heals between DPS moves if the tank gets too low. Even with the damage reduction buff, the tank can still take some pretty hefty damage to be aware of what cool downs they are using and use appropriate spells that compliment them. If all else fails, better be safe than sorry and letting the tank die.

Also be aware that one type of mob in the dungeon can apply an increased damage received debuff that should be removed ASAP. (I think it's the mole type mobs that can do that).

This is just a general guide line to how I do DD farm runs and your strategy will most likely change based on your own play style, group composition, and player skill and comfort level.

[Magistrella]

Also be aware that one type of mob in the dungeon can apply an increased damage received debuff that should be removed ASAP. (I think it's the mole type mobs that can do that).

Anything Ghislain said except that its the anglerfish that use the vulnerability buff. And it doesnt decline so it will always be 15s long :/