I learned a great strategy from my friend recently. It is the smartest one I've probably heard yet. Too bad it's too common...
1. Get a Rattata at the lowest level as possible.
2. Give it a Focus Sash.
3. Teach it Endeavour and Quick Attack.
For example, if you were fighting a Lv. 100 Infernape, it will obviously attack you first. If it hits your Rattata with an attack like Flamethrower, you will have 1 HP remaining because of your Focus Sash. When you use Endeavour, it brings your opponent down to the same amount of HP you have. The Infernape will have 1 HP left. Quick Attack will hit first, so use it when Infernape is 1 HP to defeat the Infernape. And there! A low level Ratatta has defeated a Lv. 100 Infernape!
Credits to my friend for teaching me this strategy.