Arachnophobia is a Naxxramas raid achievement. Completing this achievement is criteria for the meta achievement 16px .
A raid in 25 man Tier 7 or equivalent should be able to do this achievement. To start off, clear all the trash in the first room before Anub'Rekhan to ensure that you don't have to worry about it later. The key thing to remember is KEEP MOVING. Don't worry about badges or loot. The timer starts when you KILL Anub'Rekhan and ends when you KILL Meaxxna. When dealing with trash groups, massive pulls into an obvious AoE kill zone are best. If you can pull off "Momma Said to Knock You Out" (Killing the Grand Widow without despelling her soft enrages), then you should be able to get this achievement.
The main thing to remember is hold off on looting till the end, then coming back through and looting.