As someone who has played about 4000 hours of the sims 4 (It's a problem, I know), I probably would have gone for Gus's strategy of using the fireplaces, but I would have made the building smaller and used multiple rugs across the floor/underneath the fireplaces to help it spread. Probably would have also gone with the cheapest fireplaces and the cheapest rugs because they have lower fire ratings and would be more likely to catch on fire. Barbara's strategy of starting fires was good, but doomed after the first fire went out. The game has a reset timer so that you can't have two fires within a certain amount of time (I think it's 5 sim days). So once the first fire went out, she had no chance of burning any of the others to death. She could have then tried to starve her Sims to death if she just wanted to keep her current build, but it would have taken at least two sim days because of all of the food she had cooked already and she would have had to get rid of that food immediately. She would have also probably had to get rid of the stoves because the sims kept cooking Chris's strategy was solid, but it has become harder to drown the sims since they added the ability to get out of the pool without a ladder. Blaine's problem was switching strategies too often in my opinion. Turning autonomy off would have also helped some of their situations, but I'm not sure if that was a rule they had put into place or not.
Edit: Someone else's comment reminded me that fireworks are a thing in the Sims. That would have been the quickest and most sure way to start a fire. Basically all they would have had ot do was build a small room with one of the mortars in the middle and light it with all four of the people in the room.