Watch your npc chatter, they will announce something like "Freeze Villain" and that means they're coming after you. They spawn when you pass specific spots on the map. So, if you zip to the bank, you'll have every spawn you pass coming after you, including the bank door spawn. I watch for the message, then wait for their arrival and take them down before moving on. Once you've defogged part of the map, they won't spawn again in the defogged areas.
When you solo a mayhem, you get 2 longbow per spawn. However, when you add teammates, you get more for each member. They will follow you in and out of doors.
One thing I like to do, I have my npc chatter in my main chat window, is after I get a "Freeze Villain" I will spam my battle cry. That way there's a marker between each spawn message.
You will also get a longbow spawn each time you enter a side mission and the bank. So, I wait at the door to take them out so they're not added to mix inside. When you break the vault, you get a spawn of police, longbow with hero, and then more police after you defeat the hero on your way out.