To get this achievement, ignite a Spy using the Disguise Kit taunt as a Pyro. Here's how to do it solo:
1. Use the "Engineer in Space" map from the Workshop. Subscribe to it and relaunch TF2.
2. Go to Main Menu > Find a Game > Create Server > Select map "(UGC) achievement_engineer..."
3. Join team Blue as a Pyro.
4. Open the dev console ("~") and enter:
sv_cheats 1; bot -team Red -class Spy -name Bot01; sv_cheats 0
5. A Red Spy will appear. Enter in console:
sv_cheats 1; cc_bot_selectweapon Bot01 3; sv_cheats 0
6. The Red Spy will select the Disguise Kit. Then type in console:
sv_cheats 1; bot_command Bot01 taunt; sv_cheats 0
7. Repeat until the Spy pulls out a cigarette, then ignite them to unlock the achievement.