Giygas

I used a Simple Swoobat. Took out everything on his team sans the Skuntank in a single hit.

After you see the scene where Corey falls to his demise, you'll be able to access the Lapis Ward.

Castle stays dark unless you use the healing machine inside then lose a battle. Then it's light until you leave it. Also, like everybody said, this tree is located somewhere on the right side. What happens after that is a little more complicated.

I joined Magma. Thankfully my starter was (though probably a poor choice in the long run) Venusaur. Those poor Sharpedos didn't even stand a chance.

I've managed to complete level 8 on two occasions; it was heavy on guesswork. However, I've made a lot of accidental flips on a Voltorb when I hastily attempt to complete a level. Lesson learned.