Sharla + Riki + Melia = OP.
I used a lot of Dunban + Riki + Melia, which is just as OP. The only bad thing about Riki is despite having tons of HP and lots of good skills, he can't hold aggro for crap.
Although, you could probably sub in 7th for Dunban. With the right skill/equip combination, you don't even really have to use many skills for 7th, with that uber Haste + Auto-Attack. But I prefer Dunban due to better banter and tankiness.
Reyn is still useful when fighting groups. Dunban is only a good tank for single enemies.
Sorta. Reyn excels in aggroing everything around him and using quite a few AoE attacks to keep them in focus. Dunban excels in Evade Tanking, so even if he does have lots of stuff on him, he's likely not getting hit (assuming he's set up properly). However, Dunban is considerably frailer than Reyn, so take that if he's hit, it'll probably hurt.
Although with Dunban's skills, he's either grabbing aggro from everyone else and tanking, or dropping it and being an attacker. Dunban has very few AoE attacks, so he is somewhat better vs single enemies. Both have some good auras depending on what you wanna do with them though.
I'm still early in the game (just got Sharla) but I think I am missing something with the chain attacks, because they don't seem to be anything special except for popping off some emergency heals (I'm probably doing something wrong, will backtrack in this thread for info).
Also, does anyone else have issues changing targets when there's more than two? I've been playing as Reyn because the AI doesn't seem to be able to tank competently but sometimes when one of the other members pulls aggro I can't seem to switch my target to the loose mob... even if the mob is positioned in front of me it sometimes feels like the targeting system jams up or something or outright refuses to target the mob in front of me.
In the early game, Chain Attacks are just good for forcing Topple or other effects. Later on with good Affinity, they're downright beastly and dish out some major damage. Think my best combo resulted in about 130k+ damage. I'd have to check.
As for targeting, what controller are you using? I know for the remote, you have to hold Z and move the cursor. (I think that's right...?)