The Nightmare Frontier is quite nasty, but before you can leave, you'll have to face a very freaky boss. You've seen the enormous spider creatures clinging to the buildings in Yahar'gul, and now you'll fight one: Amygdala.
Amygdala is huge, which means you can attack her just about anywhere. Unfortunately, her limbs have very high defence - you'll have to aim for the head when she leans down. This means dodging her many swipes and palm pounds, rolling directly under her when possible. At a long range, she'll fire a laser beam at you. You can dodge it by dashing.
After taking enough damage, Amygdala will draw four giant weapons, one for each hand. Use the same strategy of rolling under her slashes, but be aware that her attacks will hit much harder during this time. Stay on the move, attack her head when possible, and you'll conquer the horrifying spider lady.