Remarkably easy - here's how:
First, build a city as usual. Make sure you build a Mausoleum and buy the Booby Traps upgrade. Then, you'll want to destroy all sources of fun, such as restaurants; all needs facilities, like churches, clinics, farms, etc.; and select every controversial edict, like Prohibition and Nuclear Testing, but especially Conscription and Inquisition. Fire all of your soldiers and block the job slots.
At this point, your happiness % should be going down like crazy and a steady stream of rebels should be appearing. Once you get to about 20 or so rebels, issue the Lure the Rebels edict. They'll attack the mausoleum and fight El Presidente, but 10 or so should survive. Once they blow up the Mausoleum, it will explode and take out all of the nearby rebels (and occasionally a civilian). The achievement should pop around the time of the explosion if enough rebels are killed.
UPDATE:
SilentRich69 passed along a quick way to do this via a challenge map (also, see the video below):
Okay short and sweet way to do this.
1. Go to challenges and click meet the rebels.
2. When you start build a mausoleum with your starting cash.
3. Then build several farms just to stabilize your tiny economy.
4. Wait until you get a message at your Palace from CEO Keith Preston and take his offer of 100 rebels and $50,000.
5. Use the $50,000 to buy a trap for your mausoleum and then build a ministry so you can get a defense minister.
6. Issue the lure the rebels edict and let them wipe your men out they should all gather by the mausoleum and be blown to bits.