Destroying the world as we know it, a Handbook

[Elemental]

The tribble idea is excellent. Tell the DM you want to use magic to create a CR 1/8 creature the size of a large rat. Tell him that it breeds fast. And ask him if you can create a new 3rd level spell "rain of fluff." This spell creates 1d4+1/level tribbles (maximum +10 tribbles) who lie around and do nothing.
Tell the DM that they do breed quickly, and when his back is turned drown the world in infinite tribbles.

Or you create hundreds of demiplanes using Genesis, populate them with thousands or millions of tribbles, and position the gates to the tribble planes in the sky at strategic points around the world. Then open them all at once, drowning the world in tribbles, who create more tribbles, who create more tribbles. And the World dies!

And a Klingon armada drops out of warp in orbit and obliterates the surface, thus exterminating the tribbles. They leave, and you get a free planet to do with as you please.