Damage Reduction

Hi Guys,
Currently i'm new to Toram and Halberd stat. Been searching a good combo skill and equip, just wandering if you fight boss like Mino, Roga, Warmonger etc with different level of difficulty, does the damage reduce due to their stats?
For example, I fight Mino on Nightmare Mode -> my burst combo only deal max of 25k+, while I fight Mino on Ulti Mode, my combo only deal 15k+.
My eq is contain of crit rate and crit damage. And i don't use p.ray so sometimes it do crit, sometimes doesn't
thanks and really appreciate your help

 

Yep the defence of bosses increases as their difficulty increases.
Not only that but their dodge becomes so high that all your attacks will start grazing (graze = reduces stability by half) so thats another reason why damage drops on higher difficulty.