My opinions about Xenoblade AI:
Shulk:
Is pretty good at dealing out damage and stuff. However, his Monado art usage is just wasteful. This is not so bad later in the game when, if you warn him, his talent gauge maxes out for you. You still need to waste a party gauge bar in that instance
.
Reyn: Could be better at getting aggro, but he usually suffices. The issue is he does not know how to use guard shift correctly. He only activates it when he's at low HP, and does not activate anchor chain (maintains aggro) beforehand. So, he loses his aggro. Then, he dos not deactivate guard shift? He's just stuck there doing nothing? Yep.
Sharla:
Ick. Bad AI. When a party member has a status condition, she waits forever before using cure bullet (when it's clearly active). Furthermore, when the entire party has a status, instead of using cure round, she will probably just cure bullet one person. When it comes to healing, she often heals the wrong person. If your tank is at low hp, and you have only taken a slight bit of damage, she is more likely to heal you than your tank, for some reason. It is annoying. Oh yeah, and she always lets her ether rifle fill ALL THE WAY before letting it cool off, so she often is not even available to assist. Seriously, if your team is relying on Sharla, I recommend controlling her if possible. You will be surprised at how much easier things become. Playing as Sharla is interesting, as you sort of become the commander, in a way.
Melia:
Summons elementals in seemingly random order, may do something with them, or not. I think her AI is more built for a support role than a DPS one, but I'm not sure if the AI is capable of knowing exactly what elementals to have summoned or not.
Riki:
If equipped with ether attacks, he will not use them often at all. He would rather auto attack. This makes it hard to have AI Riki tanking. He's also a little too happy to use You Can Do It, which is annoying when trying to have him avoid aggro. So, what do you do with him as AI?
No opinions about the AI of any other characters.
I usually like to control Riki or Sharla while tanking (btw this is late-game), and have Shulk and somebody else on the team. I tend to face enemies far above my level, and Riki and Sharla are both able to maintain aggro against such people. Sharla is actually a pretty good tank with correct gems, correct arts, and correct skill links. Heal Round and Cure Round both give her a lot of aggro, and from all enemies around her, so that's covered. So, anyways, if I get a vision I usually sacrifice a party bar to Shulk for the Monado Art. Uhhhhh yeah. The last person is usually just dps or something. **I don't topple spam
** Indeed, I beat Avalanche Abaasy with no topples or chain attacks. I did that controlling Sharla, having Riki tank, and then Shulk doing damage and having visions (night vision gems for accuracy, of course).
I might also control Reyn if the opponent is within level. Ever tried offensive Reyn? It's a little fun to test out and see how high of damage you can get from a single sword drive paired with both Berserker aura and Magnum Charge (50k+). Offensive Reyn still has pretty good defense, too, so he can be tanking at the same time.