How can I get up to the hole in the giant's arm?

You've probably already thought of causing the lion some pain so he'd put you
where you need to be.

And what you probably haven't thought of is that the more pain you cause the
lion, the higher up you'll fly.

Of course, you need to throw the stone down on the lion when one of the
goblins is standing on the lion's head.

Try throwing the stone.  Notice that the goblin who picks it up will pause
for a few seconds.

And during that pause, the other goblin can take the stone from his partner.

So, for example, pick up the stone (F).  Take the stone from Fingus's hands
(W).  The stone will move up to the next tier of ground.