Depends on your desired level of despotism. Could be achieved with 'shoot anything that moves' strategy. Asymmetric response with artillery and airstrikes on any hostile target.
People have tried playing exceedingly dirty in the past - colonial powers, especially. It doesn't usually work. Ultimately, unless you're willing to build your own infrastructure, you need to rely on local infrastructure. That requires supporters who won't try and sabotage you at every turn, and report everything you do to the enemy. Building your own infrastructure takes lots of people, money, and time. Successful asymmetric warfare works exactly the opposite way to killing all your enemies - it works by taking a group of your enemies, then convincing them you're the better bet.
There are different approaches, it depends entirely on what your strategic goals are for the city. You can drop leaflet propaganda informing citizens to evacuate such as they did in Aleppo [1] and many other places. The premise is simple: give harbor to combatants and you will be treated as such.
Here's a great site that has some leaflets from the Iraq war: [2]