Nightmare difficulty becomes available after you beat the game on the hard difficulty level, you will have to be more strategic with enemies and not try to charge enemies as you take a lot more damage from zombies and can die fairly quickly.
General Tips for beating Enemy types: Use a hit and run approach. Use the dash (A, then hold Y) to charge the leaping slam move then use the B, X, X, X move to quickly execute enemies & recover health. For clown enemies, after you do the leaping slam move, back off a little as they will try to kick you, followup with the B,X,X.X move to do damage, rinse & repeat until you can execute them.
For electric bride bosses, knock down her shields (B,B move) then do a X,X,X or B,X,X,X move. Try to also harvest the various elemental powers/weapons from your enemies & use them against them to deal more damage. Use the spinal zap weapon/flail you harvest from the electric bride against the acid spitting bosses to crystallize them and then destroy the crystals by walking up to them & holding the Y button for a rocket punch attack.
Try to return any of the projectiles that enemies spit/shoot at you and take advantage of the slow motion sequence to deal extra damage to enemies when you return projectiles.
If you find a section is to hard, consider using your Blood Lust power to deal major damage to enemies. You can farm blood lust by executing enemies, then dieing & continuing from checkpoint until you've executed enough enemies to renew your Blood lust power. If people are having difficulty in certain sections, message me and I will try to give you strategies for passing that section.