O.K. this glitch allows you to make the fight look easy(it WILL be easy)and the glitch itself is also easy.First you have to start the fight by getting on the platform above the water in then serris will start attacking you probably will hit you.See the wall climb on the right hand side of the fieldClimb to the top.WhyBecause that's the ONLY place where serris won't attack so just keep shooting him until he turns into an
X-parasite,then all you need to do is shoot him with missles.(NOTE:when serris turns into an X-parasite you might want to get off the wall)