Several hidden timers in this mission. You have to secure the two civilians asap, in order to unlock the first reinforcements quickly. I prefer calling W6 unit over W5 because portable cannons are OP against the kind of fire you have to face. Meanwhile, you have to start extinguishing the fire but first you must let the fire spread to at least one dwelling, in order to trigger the little cutscene and the new objective (extinguish dwellings). Completing this objective will allow you to progress faster in the mission (10 seconds after this objective completion, the gas leak event will occur). Once the gas leak has been managed, you have to extinguish every fire to complete the mission. However, 4 minutes after the gas leak event, the plunderer event will start, and thus delay the mission ending. In order to skip it, the fire must be extinguished quickly. It is key to notice that the fire on the top part of the map will fade on its own after approximately 09:45 in-game time. Unless you are able to extinguish the top of the map yourself, as well as dealing with all the fires and the events in a very decent timing, I guess the perfect time following this strategy would be a 9:59 with two fast unpauses.