How do I create a magic that...?

[Sigma]

...allows the player to decrease the number of enemy encounters he or she has?

[:)]

well you can totally disable it, but with knowledge of conditional branch you can make it "decrease" the amount of encounters.
to get rid of it:

under skills, go to common event. then use event option "change encounter" turn off.

[Sigma]

And this is for RPG Maker 2003.

[:)]

then nvm.

[Esmeralda]

MAke a skill operate a switch, you can call it whatever you like, then go to common events. Make it 'Paralell Process' and the condition switch to the one you just made. Then make a Fork Condition (Disable the 'Else' button) and make it decrease the encounter rate (Can be done by the default menu commands).