Easy with someone who's willing to help you boost.
Had a friend start an arson mission as a police officer and get too close to a burning dumpster until their health fell to 0. Simply heal them 10 times as a medic.
Other ways work too but this seemed to be the most reliable. Firefighters, as of the current version, can't take smoke damage so that's out. Being shot as an officer is slow because the attacker will usually flee. Being hit by weapons, fists, and cars also currently do nothing to other players.