How do I create a magic that...?

...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.

And this is for RPG Maker 2003.

then nvm.

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).