Auto-heal is one of the best effects a spirit can have.
Hell no. Auto-Heal recovers 2.5% every 5 seconds. That's 0.5% per second, normalized. Great Auto-Heal does double that (5% every 5 seconds). Most of the Ace's and Legends can easily overwhelm that so it'll only shave maybe 10% off their DPS. Stamina Up or Critical Health Healing are almost always better, and take up less slots than Celebi. Auto-Heal is
weak.
Even discounting the hazard immunities (Fog, Lava Floor, Screen Flip, etc), I can think of several effects that I've found far more useful than Auto-Heal. In no particular order:
- SUPER ARMOR
- Giant
- Stamina Up
- Critical Health Healing
- Critical Immunity
- Franklin Badge Equipped
- Assist Killer
- KOs Heal Damage
As to the Phantom Thieves spirit battle specifically, the AI is absolutely
horrible at dealing with super armor. Golden Mario, Polar Bear, or Regigigas are God-Tier supports here. There's a Primary spirit from the Mega Man Franchise (I think it's the one X enhances into) that also gives super armor effect. Then just pick a heavy and smash smash smash all the fish in the barrel. I favor Ganondorf with the giant-ass sword.
Or pick Little Mac and spam neutral B. It has super armor on charge up and the AI almost never respects it. Hit them a couple times with tilts or dash attack to get like 25%, then bust out the flaming fist to send them flying.
Seriously, once you realize just how stupidly broken super armor is against the AI, a lot of these legend fights aren't even fun anymore.