Here are the gotchas for Northern Beasts as best I can remember them.
Phase 1 - Gormok
This is a 2 tank fight, with tanks taunting off each other when they get 2-3 stacks of impale. Everyone else spreads out and smashes him a lot. There are annoying little creatures called "snobolds" that are often called out as Kobolds over vent. When someone has one, ranged need to DPS it down. The kobolds stun and interrupt and generally make life annoying for the person they are climbing on. Apart from that, it's tank and spank.
Phase 2 - Not one, but TWO Jormungar Worms
As their names suggest, Acidmaw and Dreadscale throw out poison and fire respectively. Also, one of them can move, and one is stationary at any one time.
So, 2 tanks - one on Acidmaw, one on Dreadscale. One of the worms will be "mobile" and the tank with the "mobile" worm needs to kite it around, just like Grobbulus as they will drop the same green blobs of goo. The other worm will be stationary. Periodically both worms will burrow under the ground and swap who is the "mobile" worm. Their abilities remain unchanged. So in one phase, Acidmaw might be moving and dropping goo, and Dreadscale staying still. They will both then burrow under the ground and come back up. This time Acidmaw is stationary and Dreadscale is moving and dropping goo. Just make sure your tanks remember this.
Now, the real trick to this bit is fire and acid. At some stage, Acidmaw will breathe a paralytic poison all over the raid, and Dreadscale will breathe fire in an arc in front of him. For some reason beyond me, the fire decurses the poison. Yeah, I know, weird but true. OK, so for 25 man we get an offtank in front of Dread to take the fire debuff - it is their job to run around like a dickhead and decurse all the people who are bright green. If the tank of Acid got the green, the OT runs there first. It is the job of everyone with the green to head toward the OT as soon as they get the green. The longer you have the green, the more paralyzed you become until eventually you cant do anything. In 10 man, it is the green people's responsibility to get themselves to the tank with the fire buff (as there are not enough people to run it around)
DPS the two worms down evenly to get the achievement (not as hard as you might think)
What about the Zerg Acidmaw idea?
There is another school of thought which is much more of a Zerg strat, but may not prepare you for hardmode as well. Still 2 tanks, but all DPS zergs Acidmaw. Why Acidmaw? so the poison is removed from the fight as soon as possible. If you are having issues moving the flame around the group, you may want to try doing this. Healers will have to work a bit harder on the tank left on Dreadscale, but I hope that should not be an issue for you.
Phase 3 - Icehowl
This guy is not as straight forward as you might think. First up, tank and spank. Hold him in the middle, try not to stand in front of him as he freezes people.
At some point Icehowl will crack it with the raid, and do a massive stomp, stunning everyone and throwing them to the wall. Icehowl will then give someone the evil eye - note who that person is! He will then back up, and while he is backing up you are freed from the stun and are so scared that you run faster. Run the hell away from the person he is targeting. Strafe, use your mouse, anything, but do not keyboard turn or you will die if he's targeting you.
Now, Icehowl ain't too bright, he doesn't notice that you actually run away and he charges at where you were when he first gave you the evil eye. Now, if you were paying attention, he should be charging at a blank wall. If you get in his way you end up between him and a wall and you die a lot and squishily and Icehowl will grow red and enrage - HUNTERS!! TRANQ SHOT STAT!
However, if you run away and he hits a blank wall, Icehowl is stunned and takes increased damage. Smack Him Hard!! Eventually, he will shake the stun off and go back to the tank and spank phase. Tank be ready to pick him up.
As always, be careful with aggro in the increased damage phase, make sure the tanks are ready to taunt if necessary.
That's it! Pretty straight forward fight. Good luck!