Another way of getting it involving less luck than playing in a half full / full game is to try and play Gnome Bomb when there isn't many people playing (I did it mid day on a week day for example). If there is no games going on, you will start by yourself in a new one, so just wait out until someone joins. Then, when the game starts, try keeping an eye out on the bomb and wait for the opposing player to get it, follow him until he plants it and then vanquish him. I used Chomper just for the easy Burrow + Vanquish combo. When he's gone, just wait out the timer and start defuse it when the timer is around 8.