My high lvl group (all been 80 for a few months now) is Druid (TK) Priest (HL) Lock/Mage/Hunter and gundarak was the first instance I ever did in heroic. That boss is rather hard to move about on but I do the following

leave all the dps/group in the same spot (if u look at the area u fight the boss on when u first approach him as a clock with numbers round the edge then i position the group at about 5 oclock)

tank him infront of the group such that u can move both sidways and front/back while still being in range for healing/dps.

when u get the aoe it will be on the entire group so u move - initialy to the left, then forward then right, right and then back (and then make it up, group will start to get split by this point anyway)

The water elemental has limited HP so u dont need uber dps even if u can only cast for half the time, my toons now all have over 2.2k susstained dps but i have been able to down him since they were at about 1.5k or so

I guess the key thing for my group is the healer (priest) he is weak and gets one shotted often in hc however he has pretty good group heals (I use the glyph of holy nova and a castsequence CoH/nova/nova/nova macro so I can spam while moving (comes in handy any time there is aoe as u can move while healing the entire party for about 3k every 1.5s e.g. last boss in VH with the blizzard) Also the PoM is rather effective for when the entire party is taking damage.

Without the instant aoe heal i doubt i could realiable kill this boss tbh. I have tried splitting the group such that only one or two get hit by the aoe however u end up moving other ppl into their pools pretty quickley which i found to cause more problems than just leaving the entire group together - moving whenever hit by the aoe and using instant group heals while moving.

Sorry if this is a waste of ur time as it relies on having a priest, but its the only way I can fight him (as I have a priest healer in my lvl 80 group)

Toons (Hellscream UK) -

Team A (lvl 80s) Greenday (druid tank), Sviatoubiach (lock, affi), Nstar (priest, healer), Decrypt (mage, frost), Encrypt (huntard, surv)

Team B Recrypt (pala tank, 76), Inuet (shammy1 ele/resto, lvl 74), incamoo/indianmoo/escimoo (shammy2,3,4 ele/resto, lvl 73)