For those who don't want to watch a video, here's a written walkthrough:
1. You will need to go to the castle in the middle of Valeshire.
2. Fight through the castle until you find the Thunder perk. Also, beat the boss while you're here for fun.
3. When you leave the castle, use LT/RT or the keyboard to select the Thunderstruck ability to use.
4. Go to the next area to the right.
5. You'll see a bunch of enemies on screen. Move a bit more to the right so you can see 5 at once.
6. Use C on the keyboard/B on the controller to use the ability, killing everyone.