Castlevania: Dracula X/Stage 4

In order to access Stage 5' and save Annet, you must do the following:
 * Get the key from Stage 3 and keep it to the end.
 * Do not get killed ever once.
 * At area 4-4, save Maria by using the key on the door.
 * At area 4-6, use the key again on the door to skip the boss and go to Stage 5'. (The door will not open if you did not save Maria but still keep the key.)

Boss: Minotaur
The minotaur is the easiest boss in the game, as you can manipulate the AI so it never attacks. Simply run to the opposite side of the room and make sure he's as far away as possible. Approach him, and attack, then back away quickly. If he hops or moves backwards, go in for another attack. Keep your distance so he won't attack.

If you're close and above him, he'll throw rocks at you. If you are close to him and on the ground, he'll strike you with his axe (he'll also attack if you attack him a few times in a row).