This achievement can be obtained in singleplayer.
First, load the map by using this command in the console:
map sd_doomsday
Choose Red Team. Then, kick bots (if necessary) using the command:
tf_kick_bot all
Then add bot(s) back in using these commands (adapted from URL following):
mp_teams_unbalance_limit 0
sv_cheats 1;bot -team blue -class pyro;sv_cheats 0
Teleport bot to the rocket:
sv_cheats 1;bot_teleport Bot01 -1131 1062 127 0 0 0;sv_cheats 0
You can then use airblast to take the bots down near the exhaust of the rocket to be pushed into the environmental kill there. You might need to get some shots off (I used the shotgun) on your target at the end of the round for it to trigger as you using an environmental kill on the enemy, I'm not sure. See the following video as reference for where this achievement is done: