Here is a handy screenshot you can use as a guide for hitting the final beacon.

Note that this image was taken during the day time, you have to hit the beacon at night, in the dark.

You won't be able to view the trees on the left, much less use them as a height gauge.

You will still be able to see the fence, so this is what we're going to do.

You need to align the height of your aim by using the yellow container thingie that you pull your firebolts ouf of. The gap between the fence and the bottom of the container should be about half the height of the container.

Also notice that I charged the shot to 100%, hence why it is blue, you need to do that too.

My character is standing to the left of where she spawns, about one person gap from the stone slabs on the ground.

Hope this helps, the game is super easy, fun and chill, and I really enjoyed the storyline.