Go to single level play -> Experienced difficulty -> Techno Forest. You must leave at least one enemy alive while beating the final boss on stage 15 in the level. Some enemies will spawn before the boss so try to keep one of the big enemies alive. Strafe counter clockwise around the boss while shooting only when you have a clear line of sight and dash through the boss attacks. If you kill the enemy you must keep alive then simply die and you can retry stage 15 again.