Super Smash Flash

Level skip:
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In Classic, Adventure, or All-Star mode, right click at the top left. 
If done correctly, a bar will appear. Click on forward (on the bar) to 
skip to the next level. In Training mode, do the same thing and it will 
advance you to the next level. In Training mode, the last level you go 
to will be the credits. You cannot play at the credits. 
After the credits you will be taken to the main menu.

Easy wins:
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To beat 100 Man, 3 Minute, 15 Minute, jump up on the top platform. 
Keep jumping and they will not get you.

Blue:
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Successfully complete Adventure mode under the normal difficulty 
setting with Blade.

Mewtwo:
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Successfully complete Classic and Adventure mode with Pikachu 
and Jigglypuff

Naruto:
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Successfully complete Adventure mode with all default characters.

Mr. Incredible:
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Successfully complete Classic mode with all default characters.

Shadow:
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Successfully complete Classic and Adventure mode with Tails

Inuyasha:
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Successfully complete Classic and Adventure mode with the default 
Sonic characters.

Jigglypuff:
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Successfully complete Classic or Adventure with any character 
at any level.

Super Sonic:
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Successfully complete Classic mode with Sonic.

Young Link:
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Successfully complete Adventure mode with Link.

Zero:
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Complete Adventure as Megaman under at least the normal difficulty 
setting without getting a Game Over.

Final Destination stage:
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Unlock every character.

Battlefield stage:
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Complete All-Star mode.

All-Star:
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Unlock every character.

Cloud:
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Complete 100-Man Melee.

Crono:
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Complete 3-minute Melee.

Lloyd:
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Complete Adventure mode as any character after having one hour of total play time.

Luigi:
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Complete Adventure mode on at least the normal difficulty setting with Mario.

Mr. Game and Watch:
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Complete Target Test mode with every character but him.