If you're like me who had trouble, here's what worked for me.
Place a vertical tunnel above water of 43 blocks (42 - 55). At the bottom there should be a hole of water to land in. The tunnel should consist of all walls to be filled. Once constructed, activate the elytra as soon as you enter the tunnel. No fireworks are needed (for me it worked).
If you have trouble, place some ladders and keep trying. For me this was very easy as I unlocked the trophy on the first go.