Hold the back button and open up your keys to the city menu. Click on agent abilities. Turn on jet pack. Go back and scroll down to cease fire. Turn that on. It will keep enemies pretty still. Go to weapons and get a grenade launcher. Then go to spawn ai. Click on logistics ai. Spawn 30 wardens. Leave the menu open. Jump high in the air and hold A to use the jet pack then launch a grenade and kill the wardens. If you have to spawn more try and get close to the ground and since your menu is still open, just spawn more wardens without touching the ground. Shoot them again.