There are three strategies for level 21.
#1 – make matches near the earth tiles and wait until the gold nuggets in the top corners get down to the main area.
#2 – make matches to clear the tiles in the main area and collect bombs for the gold nuggets aside. Locate the bombs close enough to reach the gold nuggets.
#3 – destroy the main earth block and try to lower the gold nuggets so that the pieces appear above them. Then you can use a Lightning power-up on these pieces to get the gold nuggets.