Select "The Collapsing Condo" Challenge Dojo from the Main Map. Wave 1 will spawn from the centre, then Wave 2 from the left and Wave 3 from the right. If you manage to maintain your combo up to 100 the achievement will unlock.
Completing the Dojo will reward you with "Garmadon (Pyjamas)". If you select this new character and play the Dojo a second time you will also unlock: