Obviously you'll have to unlock Lord Garmadon (or one of his variants) before you can attempt this.
After that you need to complete The Collapsing Condo Dojo so it will unlock Free Play. It also unlocks the Pajama variant of Lord Garmadon which you can then use to get this achievement [thanks to BigWiIIieStyIe for the info]. You can access the Dojo directly from the map by pressing
and then hovering over the dojo and pressing
. The dojo is in the city section on a line stretching between Area 1: Ninjago City North and Area 8: Ninjago City Downtown.
You just need to finish the dojo on normal mode to unlock Free Play. You don't even need to get a medal and it'll unlock. [Thanks to MathGuy42 for the info] After unlocking Free Play, re-start The Collapsing Condo Dojo choosing Lord Garmadon as your minifig.
During the battle there are more than 20 Shark Army Goons that attack you and goons even still spawn while you're fighting the boss so you can easily get it here.