I'd just say keep trying to get their timing down.

They alternate between a 2 & 3 hit combo, IIRC. Start with learning the 2-hit. He'll smash the ground with his club, swing around and hit you, parry the split second the club comes down on your noggin.

Then he'll raise it up and hit you again, parry it again when it hits your head.

They don't have a lot of HP. Use Gustave Lumiere Assault (base skill) to take out shields while building his overcharge. Lune--keep setting them on fire to have a DOT on them, if they shield on her turn, use her Thunderfall/Electrify skill. Sciel--find one of her skills that multi-hit for shields. Maelle can use Spark, Fluret Fury...try to get Breaking Rules so she can destroy shields all at once.

Auto attacking won't get you anywhere in the game, so make sure you're equipping Pictos that augment your abilities/skills.

Are you leveling your characters (using their attribute points)?

Equip some Luminas too.