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
Do the latter command any number of times.
Teleport bots to the rocket:
sv_cheats 1;bot_teleport Bot01 -1131 1062 127 0 0 0;sv_cheats 0
sv_cheats 1;bot_teleport Bot02 -1078 929 128 0 0 0;sv_cheats 0
sv_cheats 1;bot_teleport Bot03 -1125 982 128 0 0 0;sv_cheats 0
sv_cheats 1;bot_teleport Bot04 -970 1075 128 0 0 0;sv_cheats 0
sv_cheats 1;bot_teleport Bot05 -936 974 128 0 0 0;sv_cheats 0
You can then use airblast to take the bots all the way to the intelligence, and destroy them. Do this 6 times in a single round for the achievement. There's probably an easier way if you know the coordinates, but this is how I was able to get the achievement without knowing a *lot* of console commands