This achievement comes down to pure luck. Your chance to randomly fire a critical rocket increases based on how much damage you have dealt within a certain period of time, to a maximum of a 12% chance per shot. Stay on the offensive to increase your critical hit chance.
You can play around with this by creating a LAN server and just killing Bots or just use regular MP
To create the LAN server:
Open the command console: ESC > Gear Icon > Under the keyboard tab select Advanced > Check off Enable Developer Console
Press the ` button to open the console and type:
map cp_dustbowl; hostname "boost"; sv_lan 0; sv_cheats 0; (I had to input this twice to be detected on my LAN for some reason)
That will create your match. Choose Soldier on the Red Team. Then type the following command:
mp_teams_unbalance_limit 0;
and then to add a bot:
tf_bot_add blue;
Repeat to add more bots