Instructions
Equip all of the characters in your party with weapons with the "Catch" ability. You can purchase these weapons at the Monster Arena in the Calm Lands.
Go to the Mi'ihen Highroad.
Capture a Mi'ihen Fang, Ipiria, Floating Eye, White Element, Raldo, Vouivre, Bomb and Dual Horn. All you have to do to capture a monster is kill it using a weapon with "Catch" on it. Defeating the enemies on the Mi'ihen Highroad is trivial by the point you can get "Catch" weapons.
Go to the Monster Arena.
Re-equip the weapons you normally use for combat.
Choose Kottos to fight from the monster list in the arena. He is unlocked after you capture at least one of each of the monsters listed in Step 3. It costs 6,000 gil, the currency of the game, to fight him.
Use a Power Distiller on Kottos. This changes all of the items he normally drops into power spheres.
Cast "Protect" on each of your party members. All of Kottos' attacks are physical, so this negates a large part of his damage.
Use "Sentinel" with whatever character has the highest defense. This causes Kottos to attack only the character using "Sentinel" and further reduces the amount of damage Kottos can do.
Use your most powerful attacks with your other two characters. Kottos has no special weaknesses or resistances, so there are no specific attacks that are more useful for defeating him.
Kill Kottos with an attack that does at least 15,000 more damage than his remaining HP. It is useful to save an "Overdrive" attack on one of your characters for this. This causes an overkill to occur. Kottos usually drops 40 power spheres. On rare occasions, he only drops two power spheres.
Repeat Steps 6 through 11 to get as many power spheres as desired.