Mission Tree: http://www.gamefaqs.com/console/xbox360/file/944262/58302
note: to obtain the Kiku blade weapon which does stupid amounts of damage, complete mission 24. Defeat Red Rum & Starka on normal.
Easiest mission to find this guy on:
M21: Defend Megalis
Normal: Aircraft, Aerial NORMALs, AF Eclipse, Megalis [protect]
Equip your favorite small weapon: machine gun, rifle, laser, whatever. Then equip a KIKU blade on the other arm and whatever reasonably accurate weapon you want on the back (gatling cannons are a good choice). Start out by eliminating the aerial NORMALs because they're annoying and their lasers sting. After that, just shoot everything that's flying around. After AF Eclipse enters the area, there should be 3-5 additional targets buzzing about Megalis, so kill them. Then zip right up to Eclipse and KIKU it right in the nose (or anywhere else). If you took care of the NORMALs first and were careful not to get hit by the AF's main cannon, you've S-Ranked this mission.