Firefighting Simulator - The Squad

The fastest way I found is

Explosive Hot Dogs

The Mission takes less than a minute if choose "Start At Mission" and you have only one small fire to extinguish and one victim to rescue.
Grab the fire extinguisher and run around the truck. Initiate the second/green teammate to rescue the victim since he`s closest and then extinguish the fire.

Another easy way is the "Subway Renovation". With the randomizer at "1" you have 7 victims to rescue and just some easy to extinguish fires. Just connect the supply line, get the pry tool to open the door to the fuse box. This one altough takes about 8-9 mins without the ride.

And for anyone wondering: you don't have to rescue the victims by yourself. Victims rescued by AI teammates do count towards the achievement.

After you have reached level 40 and completed all missions if you are like me and need to farm the rest of the achievement load up the level

" The Office burn-out "

This level only takes a few minutes and it has 4 sometimes 5 victims to save and only a few small office fires. I have found this level to be the fastest for getting the achievement.