As a summoner, you start off with Carbuncle (the Megabuster). You then proceed to fight all the avatars (robot masters) and slay them one at a time. When you kill one, you take that avatar's power (yay new weapon) and pit that weapon against his brother (Robot Master weak to that). So you would get Titan and pit him against Ramuh, etc etc and you win.
Then you get all the avatars and you have to fight Fenrir(Dr. Wily) in Toraimorai Canal (final dungeon). What do you have to do in every final dungeon of the Megaman series? Fight all the Robot Masters again! So in order to fight Fenrir, you have to defeat all the avatars once more!
Game makes me think of my old Nintendo Megaman 3 days. xD