Level 1- 1 shot Shoot the ball in an arc so that it hits the captain and knocks off the crewmate off the barrel. May be hard, but becomes easier with practice.
Level 2- 2 shots
Shoot the Viking on the top of the hill. If done right, the ball could hit the Viking below, knocking him off the dock. Shoot the remaining Viking off the boat.
Level 3- 2 shots Rockets aren't needed for this level, although they're useful if you miss. Shoot the guys on the boat (Be warned: getting the knockback necessary to knock both off can be tricky). Shoot the last guy on the jetski.
Level 4- 2 shots Shoot the jetski guy off into the kid on the floatie. Both will die. You can shoot the last kid with a rocket, although you could arc it so the ball bounces off the tree and knocks him off. Can be hard to pull off though. (This method is recommended due to shaving off 3-5 seconds waiting for the bar to deplete.)
Level 5- 1-2 shots Shoot the guy in the middle. The ball should fall down and knock the lowest guy off. Try to land the next shot in between the last two. The rebound should knock both off. It's possible to beat this level in one shot with a grenade, but the knockback needed to get the guy in the back makes it unlikely.
Level 6- 2 shots Aim for the crewmate on the barrel. The knockback should get the other one. Shoot the captain off the mast. Rockets are an alternative if the angle's too hard.
Level 7- 2 shots Ok, here's where I recommend rockets, since the knockback begins to work against you. Hit the first "savage" off the boat. Use the Rocket to finish the others off.
Level 8- 2 shots Rockets are needed for this one. Shoot the Viking off the boat, then use the rocket to hit the Viking in the middle. If done right, the patented Raft Wars knockback should kill the others, and the Rocket will one-shot the middle Viking.
Level 9- 3 shots Rockets could be used due to difficult shots. Hit the jetski, then eliminate the one in the back. Try to get knockback for the two left, although a Rocket could get them easily.
Level 10- 2-3 shots Rockets could be used, although a Grenade can save you a shot. Hit the jetski, try to knock out the one on top, and arc the last shot to rebound and knock the other two off. A Grenade can be used to hit all three in the back.
Level 11- 1-2 shots Most players can beat this level with two rockets, although time can be saved by shooting via rebound.
These tricks can save you up to 20-30 seconds if done right.