In chapter 3,4 and 5 my teams almost always come down to the same 3/4 compositions
Encounter Aggro: Simon, Varia, Robin, Riala
This team is mostly used for misc. encounters, and can win in one turn against 80% of normal enemies, if you know what arcane magic is effective against which enemies. The team also outspeeds pretty much anything, but Varia is meant to be the slowest member so her furious strikes can clean up those enemies that are barely standing. You rarely take more than 1 or 2 attacks in each battle with this team
Boss aggro: Simon, Robin/Varia, Aka/Vhala, Qum
You can clear most boss encounters in 2-4 turns with some variation of this team. Qum buffs attack and Simon supports to keep the team alive. If fighting a solo boss, then aka disables it. If the boss is multiple enemies, then Vhala debuffs them. Varia is my preffered DPSer, with Servants Frenzy+two stacks of ATK buff+ Blade Dance, but if the enemy is much weaker to magic than physical damage, i bring Robin instead. Still, with these buffs I have seen Varia solo bosses in one turn, with a few lucky crits (Like the essence of corruption in the Stineford mine)
Tanky boss team: Simon, Robin, Vhala/Riala, Carina
Rarely used, but vital for some of the hard, optional fights, like the Skittering Nightmare or The Knights of Terror. Carina buffs, Vhala debuffs and occationally uses anti-sex, while Robin uses lances. Concidering the difficulty of these fights, Simon usually just tries to keep the team alive. If the enemy is immune to debuffs, i bring Riala for some additional dps, as Varia is a bit too scrawny for enemies this tough.