Party help. Just starting out

Hi

I started to play the game two weeks ago. My hero was Ken. I read about the units but I am still confuse about what heroes I use. I got:
Luna
Kiris
Kluri
Achates
Montmorancy (yesterday)
Carrot
Dominiel
Mercedes
Corvus
And playing the event I got Serila.
I am using Ken and Luna together + Kluri because I read she is good + Achates for the heal but I don’t think it’s optimal to use two ST DPS. Should I substitute one of them?

Should I stop leveling Achates and level Montmorancy?
Thanks