Now that I've unlocked most of the Ascension skills that cost less than 100 AP, I've got a lot of 333 and 999 cost Ascension skills left to unlock, including skills that would remove the Damage Limit on techniques and link attacks that I want very much.
That said, I don't think I've ever had 999 or even 333 AP saved up at any given time in my 70 hours of playtime. What's the best way to rack up a ton of AP in a timely fashion?