This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
The best way is to equip your lead character with the Revenge Sword, and
use it. That way, if Ashura uses a particularly strong attack on your
lead character, the lead will counter attack and do twice the damage of
If you have the Glass Sword, use it.
Also, your mutants should use Flare on Ashura.