This Achievement is really easy.
If you have a dummy account, start a casual game with that account or if you have a TF2 friend in a game join his game. Choose the Medic and Heal until you have an ubercharge then deploy it on them.
If you are using the dummy account, just stay in the spawn so there is no chance you die. You could also communicate with your friend to do the same. To add a dummy account as a friend create a LAN server and have the Dummy account Join and you can initiate a trade with the dummy account and open their profile from there and the option to add a friend will be there despite the fact that the Limited account cant add friends.