I'm no AI expert or anything like that, but I'll try to explain.
I'm guessing there's two ways. AI it's pretty much ChangeStates triggering under specific circumstances. I'll assume you know how "Random" works (if not, search around the forum, it's quite easy to understand). Let's take Jmorphman's Orochinagi:
[State -1, Orochinagi]
type = ChangeState
value = 3000
triggerAll = AILevel && NumEnemy
triggerAll = RoundState = 2 && StateType != A && !NumProjID(131035)
triggerAll = Power >= 1000 && var(20) <= 60
triggerAll = !var(39)
triggerAll = !var(16) && (var(15) < 1 || (StateNo = [1000,4999]))
triggerall = (P2BodyDist x = [0,175]) && (P2Dist y = [-120,0]) && (EnemyNear, StateType != L) && (EnemyNear, vel y >= 0)
trigger1 = (ctrl || StateNo = 52 || (StateNo = [100,101])) && Random < (10 * (AILevel ** 2 / 64.0))
trigger2 = var(7) && MoveHit && Random < (100 * (AILevel ** 2 / 64.0))
trigger2 = EnemyNear, GetHitVar(HitTime) >= 4
These two triggers are determining the part that corresponds with AI scaling:
trigger1 = (ctrl || StateNo = 52 || (StateNo = [100,101])) && Random < (10 * (AILevel ** 2 / 64.0))
trigger2 = var(7) && MoveHit && Random < (100 * (AILevel ** 2 / 64.0))
trigger1 refers to the character doing the move from neutral.
trigger2 refers to the character doing the move in a combo.
In both triggers, Kyo has a random chance of performing Orochinagi, which is determined by:
Random < (X* (AILevel ** 2 / 64.0))
The higher "X" is, the higher the chance of Kyo performing Orochinagi.
Now, you'll find that the impossible to beat JoJo characters are probably not using any sort of Random triggers that depend on AILevel; as the author probably designed the AI to always perform the optimal movements. You can apply what I explained to JoJo's characters by identifying the AI ChangeStates on them and adding these Random < (X* (AILevel ** 2 / 64.0)) to the triggers that matter. Keep in mind you'll need to get a good grasp of how the character system works, of course.