Buy a mausoleum and upgrade it with booby traps. Don't use Military Modernization. Make sure your people are not too happy so they keep becoming rebels, they need to outnumber your army. The best missions for this are No. 6 'Bickering Factions' and No. 13 'The Blind Revolution'. Once you think there are enough of them, save the game and use the edict 'lure the rebels', they will attack your mausoleum. After the rebels win, the mausoleum will blow up, taking out all the rebels around it. It works if there are a lot around, not if they already fled or are still walking out of the blast radius. Your army shouldn't be too small (I fired most, but not all), because all the rebels actually need to be in the place before they win. The trait 'Coward' might help.