This achievement does not track correctly on the Steam achievement tracker. You can check your progress by viewing your achievements via the TF2 main menu.
http://imgur.com/CTBzMhYThis achievement will take a long time. My advice is to not use the Degreaser, as it deals less damage than the other flamethrowers and thus will accumulate slower.
In addition, the flare gun/detonator does count towards this. Any sort of fire damage output as Pyro will count.
The W+M1 strategy will most likely get you this the quickest, but it also comes at a cost of being called a scrub.