You may end up spending a lot of time waiting for players to start a match or they leave mid-match. One method I have confirmed is if you start a match with at least one player and they leave, you are left to roam around the map.
Step 1: Pick a character that has a speed boost (Pea-Hyper or Engineer-Jackhammer)
Step 2: Pick up the gnome bomb and plant it at the enemy base.
Step 3: Press Respawn.
Step 4: Quickly switch Teams and choose the character with the speed ability. Immediately hit your speed boost and run to the base with the ticking gnome bomb and disarm it. If you wait until about 9-10 seconds left on the counter should also get THE LAST SECOND Achievement. Rinse & Repeat.
You'll get the notification on the bottom left of a successful disarm.