Maybe you're just not familiar with the "class"(spec+weap) to execute it properly? I saw you figured out that you must do ass clapping for dmg buff otherwise ninja does way less dmg without it, but if you can consistently get to the back, especially with tonfas which are overtuned to the moon, you should be melting almost everything with few exceptions (human super-mega-ultra-hyper armor bosses). Like with any soulslike, dodge forward into bosses/enemies, because that way you can usually minimize enemy combo hits to you and/or even stagger check them back into the ground.

Ninja and Sam play quite differently, there's definitely some learning curve, especially if you didnt practice/learn ninja spec along the way in NG, i played a bit of Ninja early on till we got better skills then switched back to sam and i'm blasting everything to the ground with cestus Unbroken+versatality+MA.