The Turtle Back is by far the hardest level in the game, so here i will show my personal strat to destroy this level First take the shell and do a casual shell jump, enter the pipe and hold Y+B, if you press left or right mario will lose speed, in the maximum height, release the shell, close to the wall try to slowdown a bit and do the shell jumps, sometimes the second shell jump will not work because mario is too close to the wall, try to evitate it. In this section you will need to use the shell to get into the next part of the level, if the koopa don't get into the shell press L and go a little bit to the left till koopa despawn and respawn. Now you are in the hardest part of the level, you need to jump in the koopa that's in the right when he is going to the right, hold Y+B in mid air to make mario fall slower, go to left, hit the koopa and in the same time go right, take the shell and kick it to the right, now go right to jump in the shell and try not to hit the pipe, if you hit the pipe mario will not have enough speed to hit the shell and will die, after hit the shell just hold right to be in the ground. Get the timing of the flying koopa and go to the midway point. Take the shell to the vines and do a shell jump, now the rest of the level is just shell jumps, there's nothing that i can do to help you. Sorry bad english :P