Destroy fortress in this level is quite hard because of surrounding planets.
Fire the Lazer bird barely seriously, just barely inside the smaller gravitational field so that it wraps around to the right without getting pulled into the larger gravitational field. Once a good angle is achieved redirect it into the bouncer on the eastern-most cookie (bottom-right). With a good angle, you should destroy be able to clip the asteroids anchors and send the lazer bird into the leftmost TNT. If necessary, fire the ice bird low so it barely skims the bottom of the small gravitational field. This should cause it to circle back into the leftmost TNT.