Thanks for the comment. I also tested this way of controlling and found it to be a bit more error-prone when you're desperate. Each has its advantages and disadvantages, I should add this simpler option if I release it for Android in the future.
Adding physics is a good suggestion, but since it's bound to change a lot, I'll think about it if I make a new grappling hook game.
Did you try grappling the boss? The goal is just to shoot its weak point (the part without spikes). Since I didn't allow the player to cancel the grappling hook, I didn't make the boss grabbable, since that will kill the player most of the time.