Enable dev console. Open the console ("~") and type the following and hit enter key:
/map itemtest
Join Blue team as a Medic and make sure Medi-Gun is equipped in the second slot of your loadout.
Enter each command line in its entirety into the console and hit enter before entering the subsequent command line:
/mp_autoteambalance 0
/mp_teams_unbalance limit 0
/sv cheats 1; bot -team Blu -class Medic; sv_cheats 0
/sv cheats 1; bot -team Blu -class Medic; sv_cheats 0
/bot_mimic 1
The bot_mimic command will make the bots mirror your actions. Triangulate and identify a midway point between you and the two bots. Walk forward toward that midway point. You should now be close together.
Select melee weapon ("3") and switch to it before switching to the Medi-Gun ("2"). This ensures both bots have it equipped. Begin charging by holding down Attack ("Mouse1").
Once you reach 100% UberCharge (status in lower right corner), look at each medic to verify that they have reached 100% charge as well. Remember the bots are mirroring you and should charge their Medi-Guns if equipped too.
Now, press Secondary Attack (Mouse2 by default) to have everyone deploy the UberCharge. The achievement will unlock.