This is at the end of the first real level (the one after the tutorial). You will be at a dockyard. Collect all the orbs - the green dots on the map help - and all the item boxes, also green dots. At the end of the level you will have to make a fight with your dragon against some enemies.
Just make combos with X and Y. For example XXXX, XXYY, YYXX or YYYY. And use the other buttons to move around.
The tower will spit out some more enemies and after they are also defeated the level is done and you will unlock this achievement together with: