Everyone's solutions are all solid ways, but here's one more method I've seen used if you're interested.
1.) Play on 2Fort with a friend being on the enemy team; have him/her be a Heavy.
2.) Meet each other underneath the bridge in the middle, under the water.
3.) As Pyro, go right inside one of the sewer entrance and face the water.
4.) Have your friend ready themselves by standing right in front of you, just out of the water.
5.) Ignite your friend with a small burst.
6.) Have your friend back into the water to extinguish their flaming body, then step towards you again.
Just remember to use small bursts so as to not burn them too much (and cause them to die).
If you're not into boosting for the achievement, following any of the suggested methods for normal matches will net you solid results more often than not. Just set aside some time and patience if you choose to go that route.
Whichever solution you use, be sure to have fun along the way!