Timing

 is the fourth level of Bloons 2 and the fourth level in the Popping Fields zone.

In this level, two groups of Bloons circle around in an anti-clockwise direction within a metal frame. One must time their shots perfectly to complete this level. It's very easy as the two 'lines' of tack bloons are fairly long. It is also the first Bloons 2 level to have Metal Blocks.

Solution
Simply wait for each group of Tack Bloons to pass by and then aim at each passing group. Each hit should make a chain reaction to that group that destroys the entire group. Aim directly into the hole in front of the Dart Monkey

Trivia

 * This level only has Tack Bloons along with metal blocks, making it the only Bloons 2 level without any normal bloons in it.
 * One dart is enough to pop one group of tacks, so the player should not have any problems.