Yeah, he isn't unbeatable, but I don't think the AI would be able to pull this off. Basically, even though he can regenerate health, just use Rare Akuma's time stop and then do powered up Skull Hadoukens, or whatever they're called. You can control them when time resumes and proceed to smash them into yuiitusin whenever you're able to. Be careful and make sure not to touch the red swirls it sends out from time to time. Time WILL resume on its own if you get damaged during the time stop, be careful of that.
Oh, and Rare Akuma's Shun Goku Satsu doesn't work on this guy. It'll hit him, but yuiitusin will break the combo. This is the only way I know how to beat him, but hitting F1 works on him too.
You can obtain him here: http://www.geocities.jp/whitelen_ha_orenoyome/yuiitusin.html
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Entei is a Fire-type Pokémon from the Pokémon series. It is a legendary Pokémon native to the...