Yeah, its really hard to isolate that portion of the bicep, although one idea, which might work for you is to do preacher curls with a full extension to the bottom position, or to add half reps(bottom portion) to almost any curling exercise. Also you can do seated incline dumbell curls. No real exercise will actually build that exact portion of the bicep, you can only add fullness to the bicep.
The brachialis is in the forearm, right under the brachiradialis. The biceps brachii and biceps femoris are in the arm (bicep).