Fluttering over the water is good practice fluttering. The sheep crab blocking the tunnel can either be damage boosted through, or killed with 3 egg shots. For the boss, you generally won't be hit while standing in the middle. However, the boss dies in ~7 egg shots if you ricochet them up into the side of the uvula. Do this rather than throwing eggs straight up at the bottom of the uvula, which takes many more egg shots to kill the boss. It is possible to kill the boss in 6 shots by shooting at the middle of the sides of the uvula, "slide down the side, aim 2 frames and shoot".
Watch the IL and use the following diagram to learn and understand how the 5 shot strat works.
The distance that the egg is from the uvula and its velocity on the frame of impact determines how much damage is done. The 5 shot strat uses a 45 degree angle and specific Yoshi positioning to get the greatest possible distance between the egg and the uvula while also ensuring that the egg has the highest possible X & Y velocity.