If you have trouble getting this achievement legitimately, you can get it alone with commands.
Step 1: Load the map "sd_doomsday" using the "Create server" menu or using the following command:
map sd_doomsday
Choose BLU team and choose the Scout to run faster.
Step 2: Disable the unbalance limit in order to have five teammates and no enemy:
mp_teams_unbalance_limit 0
Step 3: Create the bots using this command five times:
sv_cheats 1;bot -team blue -class pyro;sv_cheats 0
Step 4: Teleports the bots on the elevator using those five commands:
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
(where Bot01 to Bot05 are the bots' names)
Step 5: Get the Australium and take it to the elevator
Have fun!