Take the flying shot and pick up the 10k straight ahead, then go around the front tower and swoop down to pick up the 5k right at the entrance and go back up to blow the entire wall up (this should get you over 2x if done right).
Go left on the next flying shot and pick up the trail of 5ks to crash right into the left castle (this should bump you halfway to 3x).
Next shot should be an explosive shot, so take the right castle out and also pick up the 5k in the air and the 10k in front of the right castle. If done right with the perfect timing bonus, you should blow the whole castle up.
The final shot is another explosive shot and simply pick up the 5k and 10k pieces while hitting the front side of the castle.