Nightmare Creatures is a tricky title .What you need is a cheat that puts you on a level with the monsters, gives infinite lives and weapons, AND allows you to choose any level.
At the password screen enter: Left, Up, X, Square, Down, Triangle, Square, Down OR enter Left, Up, Triangle, Down, Circle, Triangle, Square, Down
2 : Up, Circle, Triangle, Left, Triangle, Square, X, Circle
3 : Up, X, Circle, Triangle, Triangle, Down, Square, Up
4 : Up, Square, Triangle, X, Triangle, Square, Up, Circle
7 : Triangle, Left, Triangle, X, Circle, Left, Circle, X
8 : Triangle, Right, Triangle, Up, Circle, Circle, X, Square
9 : Circle, Triangle, Triangle, X, Circle, Circle, Square, X
10: Circle, Circle, Triangle, Right, Triangle, Up, Down, Down
To Beat 1st Boss:
.All you need to do is dodge to one of the five pillars which surround the pit and slash at it until it falls on the boss,before moving to the next and repeating the process. Once all five are down, the fire will stop and he'll lunge at you instead.Use L1 and R1 to dodge when he does and quickly hit him as his head misses you.