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You can switch to the Zero Suit at any time during a match without needing a final smash. You can also start with the Zero Suit if you hold the proper button when starting the match. When I heard about Zero Suit Samus, I was thrilled to find another Sheik-like character to play with. When I realized that you have to use a final smash to drop your armor and use the Zero Suit, I realized how hard it would be to make full use of this character. Considering how rare it is to actually get a smash ball, the Zero Suit seemed like it would be seldom available...Until Now.

Switch to Zero Suit Samus during battle:

Press Up Taunt, Down Taunt, Up Taunt Quickly.
If you use this button combination quickly enough, you will drop your armor and switch to Zero Suit Samus.    

Start a match with Zero Suit Samus:

On the character screen Select Samus and hold the appropriate button before the match starts:
Wii Remote/Nunchuk: Hold the Z Button
Classic/Gamecube Controller: Hold the R Button
Wii Remote Sideways: Hold the (-) Minus Button
If you hold it down just after the match is selected until your character appears, you should start the match already in the Zero Suit.