Get a low level Pokemon with these stats.

Recommended Stats:

Lv.1
Item:Focus Sash
Moves: Endeavor, Quick Attack and any other two moves.

Tips:

Endeavor can be taught to you by one of the Move Tutors...the one in the Survival Area.
Starly can learn Quick Attack while leveling up...

Reccomended Pokemon: Starly, Rattata

Battle Outcome:

Opponent attacks.

Your Pokemon survives the first hit with 1HP because of the Focus sash.

Your Pokemon then uses Endeavor making your opponent's Pokemon have 1HP as well.

Then, your Pokemon uses Quick Attack and attacks first, defeating your opponents Pokemon.

^^^
Typed it myself, but strategy goes to Marriland



Now, this strategy won't always work. Here is a counter against it.

A Counter (I made this one)

Have a higher level Pokemon with Quick Attack or Extreme Speed.

The outcome would probably be this.

Your Pokemon attacks.

Opponent survives the first hit with 1HP because of the Focus sash.

Opponent then uses Endeavor making your Pokemon have 1HP as well.

Then, your Pokemon uses Quick Attack or Extreme Speed and attacks first, defeating your opponents Pokemon.

I hope this strategy works for you :)