Earned the achievement yesterday, its very doable if you know the mechanics!
Here's the key: the Toxic Spider cannot use the cocoon attack on targets in point blank range. The solution then is to take a small group into a private instance (I duo'd it in my case) to avoid the danger of a random ruining it for everyone and then when starting the fight, have all party members blink or stealth in on the spider to start the fight (lest she open with the cocoon attack on the run in). From there, everyone must stay in melee range for the duration of the fight, otherwise it proceeds as normal: break bars, avoid toxic stalks, etc.
If done right with these constraints, its no harder than killing any other strong melee champ.