Monster hit physical feedback Easy!
Make monsters stagger feature
You did an excellent job with the character bobbing up and down smoothly while walking. It looks amazing.
Same easy way make the the monsters - when hit - stagger backward a few pixels the bounce back. This way they stay in place, but the hit - weapon or spell - creates a physical looking effect on them.
Monster stagger trajectory visualized: >>^^ˇˇ<<
monster sprite stagger anim = move back+1pixel, back+1pixel, up+1pixel, up+1pixel, (return to original pos before hit) down+1pixel, down+1pixel, forward+1pixel, forward+1pixel