Welcome to the guide to getting the "Thunderstruck" achievement in Besige. Without further delay, let's get right into it.

Go to Tolbrynd (The Second Isle) and find Zone 24: Midland Patrol.
As an alternative, Zone 16: The Duke’s Plea also has thunderclouds and is easier to access if you do not have a flying machine. This guide will focus on Zone 24, but I cannot imagine it would be too difficult to apply the same techniques on Zone 16.

Periodically, both of the stormclouds on this level will release thunderstrikes.
I recommend using a flying machine to float over the hordes of enemies and landing directly below one of the thunderclouds. You don't have to complete the level to get the achievement, so don't worry if the soldiers hack your machine into pieces; that's not why you're here! You shouldn't need to wait long before the lightning strikes your vehicle; you'll know when you're successful when your vehicle catches on fire.
And that's it! I hope you enjoyed getting this achievement! Don't forget to rate if you thought it was helpful, or comment if you think it could be improved!

Besiege is owned by Spiderling Studios
The Dual Copter MKI is owned by Toarteel
Thanks to Caspercaper17 for pointing out the alternative thundercloud location!