Are you playing on Challenge mode and pushing either Hollow Zero or Shiyu Defense? It will get a bit more challenging towards endgame, as does trying to hit S-ranks on higher-level resource runs.

What makes that fire team good is their ability to very quickly cause Chain attacks over and over because they generate a large amount of Daze (the yellow bar below an enemy's HP, which trips stuns (goes gray), which starts chain attacks) and being a mono-element team, also cause a fair amount of Anomaly (circle to the right of enemy HP bar) which triggers Burn in their case which is especially great for human enemies because it interrupts their attacks.

Using the switch button when the yellow cross flashes from an enemy's attack triggers a Defensive Assist. Koleda generates a decent of daze when she does a Defensive assist, which costs two slots on your switch UI element. Ben is a Defender, so his Defensive Assists only cost one slot, and they also generate a significant amount of Daze (about as much or more than Koleda doing her stun attacks properly on 2nd/4th hits). Those assist slots refill when you trigger a chain attack.

So if you're good with the Defensive Assists, the end result is triggering chain attacks repeatedly, refilling the assist bar over and over, and leaving the enemy stunned a lot for Solider 11 to capitalize on with her Burst and specials (she needs to be on field the entire time when she's capable of doing fire attacks) at the right moment when the enemy is stunned and taking extra damage and racking up a lot of anomaly even though she's an attacker.

I don't blame you for not understanding the combat. It is very poorly explained in game and all over the internet on the AI copy-paste guides.

Once you know what you're doing, ZZZ combat is super fluid and extremely satisfying.