My guild just got the achievement tonight. We used the Fusion strategy, but if you only have six healers, I'd use Ciderhelm's strategy:
After Tenebron dies, you can have a DPS that has a taunt (feral druid, ret pally) move to the position Ciderhelm is at in the video and taunt the breaths.
I don't think it's a good idea to go into the portals until Shadron is dead. You'll lose too much DPS, Shadron will stay alive for too long, and you'll run out of cooldowns to use on your MT, or Vesperon+Shadron will be up for so long Twilight Torment will kill off your raid."Hero on Tenebron burn him before Shadron lands, Clean up adds. Then send a group into the twilight to kill Acolyte's soon as Acolyte is dead back on Shadron"
Try using Bloodlust/Heroism right as Tenebron's whelps emerge from the portal. This is usually right after the second lava wave. This will help finish off Tenebron, kill the whelps faster, and it should help you get started on Shadron.
If you have people dying from fissures or firewalls, you can delegate the assignment of calling out "fissure melee" and "firewall" in vent to 1 or 2 people. Bigwigs makes this nice ping sound every firewall, and I always say "firewall" in vent when I hear it. We have a melee DPS call out "fissure melee" to keep people from tunnel visioning.
Put two AoE healers on the raid while Vesperon's disciple is up, and (as recommended in Ciderhelm's video) have your holy/prot paladins pick up the Divine Guardian talent. Coordinate paladin bubbles over vent after Vesperon summons his disciple.
Keep trying and I'm sure you'll get it!