I want to point out some tips to help you get this achievement other than a video of people unlocking it:
First, and importantly, the achievement is for defeating the Cadaverous Menagerie without anyone being hit by the blue fungus AoE. These are spawned in during the fight by the Guar. They will not spawn in when is he is stunned and in recovery. Pets will not void the achievement, so if you have a pet build don't worry about them getting hit by the AoE. Being inside the AoE of the exploding blue fungi during the fight will void the achievement, so if you get hit simply wipe and start over. You do not have to re-enter the dungeon simply start the fight over. The fungi leading up to the boss battle also do not count, it is only the ones spawned in by the Guar.
The text strategy is a little dependent on a good tank, and the below video will show you the kiting method!
Strategy:
1) Your going to prefer having ranged DPS for this fight with at least 25k+ dps (preferably some mag sorc with good AOE).
2) A self-sustain tank would be nice, but a healer will make this fight easier. Also, all players need to rock an interrupt ability (ie. crushing shock).
3) Now for the fight itself:
Have the DPS and healer go to the right where the Guar is (near the gate on the right side of the arena). The tank will initiate the fight by pulling the Bear. The tank will bring the bear over to where the Guar is, which in essence will stack the Guar, Senche, and Bear. The Guar is a ranged enemy so you will want to stack on him, and the Senche and Bear will follow.
DPS should lay down AoE on the Guar and when the Senche and Bear make it over there, cast your Ultimates on the group. You should "stun" the Guar FIRST and then take the Senche down after (I'll explain why you want to take down the Guar first). You should be able to down the Guar and the Senche before any spores spawn in. Take care if the Senche fears anyone and pounces them you must interrupt him ASAP or the pounced player will die. The tank should take responsibility of aggro'ing the dire wolfs as they spawn in (the exploding wolfs) to avoid the DPS and healer taking damage.
Once the Guar and Senche are stunned, focus all attention on the bear, you should be able to get him below 50%. The casters need to stay behind the bear so the wolfs aggro the tank instead of them. So tank should be on side of bear closest to the cliff, and dps should be on the other side. DPS him down and you should build your ultimate. As soon as the Guar spawns (this is why we kill him first, we want him to spawn before the Senche) place your ultimates down to take him down fast. He will spawn in 3 spores as soon as he recovers, but you should be able to take him down with him only spawning 6 spores or so total. By that time the Senche is coming back so focus on him and once he is down you can focus on the boss for the final 50%. If your DPS is not great, and the Guar spawns a second time don't try and take him down again, just avoid the spores and focus on finishing the boss.
Good luck!
-Damron Out