So, with the Layers, 4-Up and Animation palettes visible, we need just three frames, which you can achieve by turning the visibility of the layers off and on:
Each of the three layers must the Background layer visiblity turned on!
Here, we've made the first frame with just the "Lightning" layer visible.
For the second frame, we turned the visibility of the Lightning layer off and turned the visibility of Small spark on.
Finally, for the third frame we turned off Small Spark and turned on Big Spark.
Now we need to give the animation some timing. Click on the small drop-down arrow next to 0sec on frame one and select 0.5 sec. Repeat this with frames 2 and 3 but make these 0.1 sec. Press the "Play" button at the bottom of the animation palette to see the results.