Maybe it's just me, but having 2 dedicated Fighters in the team is the way to go. Why?
- A Level 12 Fighter can attack 3 times using 1 action. Also, they have Action Surge to gain another action - replenishes after short rest.
- So, in a single turn, a Fighter can have 2 actions, which is 6 attacks
- Pick Battlemaster subclass and choose the Riposte skill. What I do is purposefully move my fighter away from an enemy (but still within melee range) to trigger the enemy's attack of opportunity. When they miss, the Riposte will initiate a retaliation attack, at the cost of Reaction and 1 Superiority Dice.
- In terms of Feats, pick Great Weapon Master. Upon killing an enemy, you can use Bonus Action to attack once.
- Add all this up, a Fighter can have a total of 3 attacks(normal) + 3 attacks(action surge) + 1 Riposte + 1 Bonus attack = 8 attacks in total in a single turn
- Due to the high burst damage potential, it's best to ditch Sword & Shield and just go for 2-handed Greatswords/Greataxe. Kill before being killed.
- By Level 12, a single attack with a Greatsword yields an average of 20 damage (base + elemental stacks)
- If you capitalize on all 7 attacks (ignoring the bonus attack since that requires killing an enemy first), a Fighter can deal 140 damage in 1 turn.
- If you have 2 Fighters in your team (including Barbarian like Karlach), both can easily deal out 250 damage to the boss in the first turn
- This is not even counting Critical Hits, and some Battlemaster active skills like Distracting Strike, Maneuvering Strike, Feinting Attack etc. which all can deal 30% more damage.
- The remaining 2 NPCs can be Rogue and Mage/Cleric. Add in the Surprise Attacks from Astarion and some high damage spells from a Mage, any boss with 400 or less health will die in 1st turn. Worst case is near death.

With this kind of damage output, coupled with high health and high AC, what's not to like? Super robust. Morever, due to high health, Fighters seldom go down below 50% health. After a short rest, you're fully healed with replenished Action Surge and Superiority Dice, whereas a Mage/Cleric requires a Long Rest to regain spell slots. At this point, Mage and Clerics are relegated to support classes - they are there for the occasional AOE, and utility spells ie. Dimension Door, Guidance, Bless etc.