Use Unirus Life Form to get into Burst Chain Mode then unleash Soul Burst. This should speed up to the second part of the fight. Heal and use Unirus Life Form again to get into Burst Chain Mode. After one Soul Burst, Ash will go use Limit Off. Make sure you heal and keep your HPs high. If he has his swords up as his shields, then heal and defend. Soon enough, he will unleash his swords. Once that happens, Burst Chain Mode then Soul Burst again. It is best for Edge to use Soul Burst RIGHT AFTER you get into Burst Chain Mode. You can easily deal 25000 damage. You can beat Ash with three or four Soul Burst. Hopefully you got enough Unirus Life Form or your weapons have high number of hits.