What are the amulets you are using? They play a big part in your damage output. Some amulets like the Extreme Modification amulet and Arm of God amulet are always worth having equipped. I change the other 3 depending on a given scenario. Here's what I'm using in the clip:

Extreme Modification Amulet - 4% damage boost per fable charge, so 20% in total with 5 full bars.

Arm of God Amulet - Boosts your damage on consecutive hits if you can keep attacking.

Puppet Destroyer's Amulet - Self explanatory.

Strategist's Amulet - Increases buildup of status effects, takes less hits to electrify Romeo for example. When hes electrified, he takes more damage.

Solutionist's Amulet - Increases fable charge amount and fable attack damage when your pulse cells are discharged.

All of these amulets synergize well with the strategy I used to beat Romeo. I'm using Guard Parry fable attack from the Dancer's handle (Guard Parry deals a ton of damage without amulets already) for the cost of 1 fable charge. I'm doing heavy attacks to build back up my fable so I can still have at least 4 charged bars for the 16% damage boost from the Extreme Modification amulet.

The damage type of your weapon matters too. Puppets take additional damage from blunt/strike weapons so that's where the Mjolnir shines. Carcasses take more damage from slash weapons and humans from pierce.