I think I classify both Melia and Riki into an 'ether attacker' group as well. You know, comes in use for fighting enemies above your level and ether based enemies (nebulas and various things late-game in the bionis interior).
Hmm, I find it interesting that you have that party man group. I'm not saying it's wrong, it's just I only use chain attacks on occasion, so I never formed that mental grouping.
Ok, so in order to tank with Sharla you need to get her hp up. I set her up with full 50%+ hp with hp gems, and then gave her an additional +10% with a skill link to Reyn. Also gave her Dunban's 15% damage reduction and Melia 'heavy armor = good ether def' skill. She's also hooked up with max aggro up.
The arts I use are heal bullet, heal blast, heal round, cure bullet, cure round (though, I'm now realizing I should probably replace one of the two cures for heal counter), shield bullet, drive boost, and thunder bullet.
I haven't tried letting the ai use her, but I am guessing that they would not do a good job with it. For instance, the ai might decide to shield bullet somebody else instead of herself.
On activating the battle, run in and get the first auto attack or have the enemy see you, whichever comes first. Then use thunder bullet, shield bullet, and possibly heal round. Heal round produces the most aggro of all her attacks, so if the enemy is starting to switch target then that is the go to move. Use thunder bullet whenever possible... etc, you should get the idea.
edit: The thing to keep in mind is it is her healing arts that draw aggro. Shield bullet helps prevent aggro loss from enemies hurting you. But cure bullet, for instance, does not raise aggro I'm pretty sure
Using Sharla as a tank has its distinct advantages. First of all, you are not very susceptible to low tension. This is because you are not attacking as much as other tanks would be. You could also give Sharla heat bullet if you wanted, which would help even more with that. Because of this, allies won't be coming near you to encourage you, so you can keep attacks that hurt multiple people away from them (unless it's the ones that hurt a circle around the enemy).
Another great reason to use Sharla is the obvious, that she is both a healer and a tank. You can fill the other two roles with pure dps users.
Playing as the healer offers a great feeling of control, as does tanking, so being in the position of
both tank and healer offers the ultimate feeling of control.
Her healing arts aren't effected by the heavy armor, so feel free to go all out with that. There's no risk of missing your healing arts due to low agility.
Healing arts draw in aggro from everyone attacking the party, not just the main focus of the party. There is not much worry of the other enemies starting to attack your allies.
Sharla's main disadvantage is her low hp. You can use shield bullet on yourself to prevent single high damage attacks from killing you though. The only other issues I can imagine are her low dps on some teams (I would recommend allies with high dps) and maybe her aggro (I have only tried a few uniques with her so far, but have not had issues with aggro yet)
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EDIT:
I have another thing to post..
I tried to get the angles right but they are not 100%.
The first and second ones are the good comparisons. The third one looks a lot less similar to Xenoblade's Shulk than the first two, but it might just be the angle.