You probably couldn't do it as you would need to be averaging 5k damage per character every round and what is stopping you is that Tellah doesn't have the stats or spells to pull that off, and Cecil and Cid are too reliant on gear and wouldn't be able to pull it off with what is currently available even at Lv. 99. It is not helped that with the exception of the DS version, your stats pass level 70 are randomized in growth making your max level party have an unpredictable build.

IV's stats don't work like the post-V games where having high levels or even high stats guarantee even 3k damage with the weakest weapons. Your fighters are actually far more reliant on gear than you think with the exception of Yang. Even if Yang was pulling max damage he would barely break through the halfway point before the Dark Elf would end the fight. The other problem is that the Dark Elf has incredibly high stats and thus his magic will crush your party pretty quickly even with high HP, you would still be taking ridiculous amounts of damage thanks to your crappy gear and Tellha has abysmal HP, so he would probably be killed nearly every round even with max lvs. Yang, who is your best fighter by this point is also incredible weak to magic normally so even though his HP would see him through he would still die quickly if someone wasn't constantly healing and restoring status to him.

Yeah... so basically you have no chance unless their is a bug in the game's A.I. that can be exploited or you use cheats. Funny enough, you can beat the Dark Elf form but you need to utilize the Second Form glitch by having Tellah cast Reflect on the whole party and just wait to have the boss kill himself. The only reason this strategy wouldn't work beforehand is because you don't have enough rounds to burn through his HP before he uses Tornado.