Once again, you are not given the option where to start - Lambda must approach from the north, whilst Sigma starts at the south. West of both of you are Aizel, and two fighters, along with their units.

Don't worry about Aizel for now, but you'll have to work quickly. You must charge Lambda and Sigma west to meet the two Fighters, and kill them both. Try not to let Lambda herself get into combat unless she needs to, as she's weak - just let her cast Fire, and her units can do the work.

Once both of these groups have been killed, Selena and Wheeler appear to the south. They'll take care of Aizel for you, who disappears as soon as Selena destroys his troops. Now, if you've done things well, you'll be able to grab the secret tile in the water near Aizel as well.