You'll want Thundara attached to Zell and Squall's element junctions to maximize damage. Also draw as many Doubles from Elvoret as possible so you can have Selphie Dual cast Thunder/Thundara. Limit abuse is a must.

You get Thundara by learning Mid-Magic refine and converting Thunder spells into their tier two variants which should also be used to maximize Str. and Mag. stats. I would not recommend using Quezocaotl as the Boost option drags the fight out and the timer is your enemy for this challenge.