Just a few bits and peices, most "common sense" but I see so many guilds not using them. Maybe it will help you? If your guild wants any speific class / boss advice... just ask here I guess.
(I am currently 5/6 HC and DK is my main class)
Stone Guard
No need to wait for the colbalt petrifications to pop the cobalt mines. Have DK's, shadowpriests and hunters pop AMS, Disperse and deterance to pop them taking no damage.
Use 4 tanks instead of 3 on 25 man to making handling the overloads a null mechanic but having each tank drag the overloaidng mob to the others.
Feng
Dk's excel on this fight due to all phases containing magic damage they can absorb. Stack them for many deeps!
On Heroic mode, use DK's Gorefeinds Grasp on the spirit sheild to group the adds up quickly for awesome aoe.
Garajal
A simple tip to avoid the damage from the small spirits on garajal if your near a kill. Simply have a rogue use smokebomb on your raid. All the spirits will stop attacking for the 7 seconds its up. Chain them back to back for even longer immunity.
Warriors mass spell reflect can also reflect a huge ammount of damage.
Spirit Kings
Classes with immunities can avoid and remove the trapping arrows, mages can blink out of them for more DPS time on the boss.
Elegon
Dk's with glyphed pestilence can do massive damage to the sparks.
Standing on the edge of the energy vortex in the right area allows healers and ranged dps the ability to remove the stacking debuff without having to run in and out.
Will of the emporer
Dk's gorefeind grasp is excelent at controlling the emporers rage.