more useful for heroic obviously.
send a mage up as one of your DPSers. when she does smoldering devistation and its about to finish the cast, invis. you will count as being on the top of the web, and beth will not release AoE damage.
when you drop invis, pop mirrors or IB to negate damage till your tank and healer get back up.
if used on Devastation #1, Invis will be up for #3.
you can also leave your pet upstairs for devastation #2 to reduce damage to raid.
this was all nabbed from MMO-Champion btw, i had not seen it posted here before!