The trigger that loads the path to Arbiture's Grounds doesn't exist until you defeat King Bulblin and break through the fences with a boar.
Instead of making the whole trip through the Bulblin camp while fighting you can lja on top of the Camp to trigger the cutscene and skip the small key saving ~8 seconds.
The LJA is rather difficult because of the rather small frame window for a successful jump.
Note that its still faster to Jumpattack + Quickspins on King Bulblin.