Insane Difficulty: Spekkio Lufia - La Puella and her merry men - Insane Difficulty

Aside from Kureji Lufia there's also the Spekkio Lufia patch which features a new optional boss awaiting in Parcelyte Castle. You can fight La Puella as soon as you have defeated Gades at the Ancient Tower.

The Sinistrals on Doom Island are also harder (though nothing compared to their Kureji Lufia counterparts) as well as the Master and Egg Dragon.

The Egg Dragon now changes his behaviour depending on your party level. Can you still defeat him in his strongest version at level 99?

I also thought it's a good idea if there would be another Sinistral fight mid-game against Amon - just like that first fight with Gades.

Youtube - VS. Amon