You must first unlock the Editor mode, details here Builder in Crazy Machines Elements
For this trophy you need to be continuously
launching fireworks which will require a setup that allows for more than one firework to be lit and active.
Fireworks launch for about 10 seconds so you will need a few to allow for their fuses to be lit (Max of 4). The easiest to achieve this is to setup a lit firework on the ground and place a Fire Dragon
above it so that once the firework goes off they set off the fire dragon about 4 launches in. Then have that fire dragon aimed towards another firework's fuse (Only the fuse!) and repeat the setup so you will have a chain of Fireworks, setting off Fire Dragons, that set off Fireworks, etc.
Your setup should look something like this
Here's a video