Mega Man Legacy Collection

The old password system is alive and well in this game, and can be used to skip straight to the final level.

Enter the following password to get all weapons and start with 9 energy tanks:
Red - A6
Blue - A1, A3, B2, B5, D3, F4
Edit: Shi Kiseichu pointed out in the comments that adding a red in E1 will bypass the Proto Man fight.

This will take you to the fight with Proto Man. It's an easy fight, he jumps three times left and three times right, shooting at you. Proto Man has no health bar, so keep hitting him until he flees (just like his appearances during the first eight stages if you played through).

The stages of Wily's Castle are very straightforward for any platform veteran, so from here I will just give tips on the bosses. With 9 energy tanks to start with, and plenty more along the way, don't hesitate to use them as needed.

Stage 1: Kamegoro Maker - Hit each Kamegoro turtle that appears with a Shadow Blade. The machine at the top is invulnerable so ignore it and focus on the turtles. The Kamegoro move diagonally around the area, and watch for the tornadoes from the sides and bottom that can sweep you aside.

Stage 2: Yellow Devil Mk 2 - A rebuilt Yellow Devil from the original Mega Man. He moves piece by piece from left to right at the start. After he appears, he'll move right to left by bouncing in five "stacks". Slide under the stacks move from right to left, and jump over the two lowest rows of pieces going left to right. Hit the eye with the Hard Knuckle when fully assembled.

Stage 3: Hologram Mega Men - Three copies of Mega Man will appear in the same color as the player. This is a shell game where a random level has the "real" hologram. I tend to focus on one level and hit it with Top Spin. If you're low on energy tanks, use Search Snake instead.

Stage 4: Robot Master Redux - Fight all the eight Master Robots again before moving on. The weaknesses are:
Needle Man - Gemini Laser
Magnet Man - Spark Shock
Gemini Man - Search Snake
Hard Man - Magnet Missile
Top Man - Hard Knuckle
Snake Man - Needle Cannon
Spark Man - Shadow Blade
Shadow Man - Top Spin

Stage 5: Wily Machine - The first phase is vulnerable to Hard Knuckle to the cannon on the underside. For the second form, use the Rush Jet to get level with Wily and fire away.

Stage 6: Gamma - For the first phase, stand directly beneath the head and jump and fire Shadow Blades straight up. For the second phase, equip Search Snake and wait for the fists to come in from one side. The top of the fist is safe to jump on, so use them to climb to the top platform and hit Wily with your Search Snakes.

As soon as you defeat Gamma, the achievement will unlock.

If I can improve the solution, please let me know in the comments.

METHOD IS CONFIRMED TO WORK!

For Mega Man 2 through 6, there is a password system that lets you begin the games with some major milestones completed. The passwords for some of these games can bring you as far as the Dr. Wily Castle, and others can only bring you up to the point where you've beaten every Robot Master. Whatever the case, they'll end up saving you a lot of time if you're more interested in getting a game's associated achievement than playing through the game to experience it from start to finish.

Here are the passwords that will take you the furthest into each game. These passwords were created using the password generator at http://www.mmhp.net/Passwords/



In the case of Mega Man 3, this password not only skips every Robot Master, but every level thereafter with the exception of Dr. Wily's Castle. You will also start the game with nine E-Tanks and every adapter and weapon unlocked as well.