You sure picked a tough weapon to get. It's only Ivan's best weapon until around lvl 40. Then the Swift Sword is his best, because of the multiplied damage on critical hits. Anyway.
Kikuichimonji is obtained by killing the Fenrir-Ice Gargoyle team in the Venus Lighthouse.
The first notable method was formerly known as the "dark panther Method" because he was the first to discover that the opposite djinn kill will result in more exp, coins, and an increased chance of an item drop. However, this method is now more commonly called the Djinn Kill Method. If you kill the Fenrir with a Mars djinni, he will flash green, and when the battle ends, you will have more coins, experience, and an increased chance of retrieving the sword. This will
simply increase your chances during battles, nothing more.
The second method is the Reset Method.This method suggests that you go into the first room of the second part of the Venus Lighthouse. You'll know you've found the room when you see a purple Psynergy stone in the middle of it.In
this room, you can save, and reset your game. Go back into the file, and the first battle that you encounter will have a fenrir in it. Fight it, then when you are done, if you don't get a Kikuichimonji, just reset the game and try it again. This will speed up the process a bit for some, but it also takes some of the honor out of getting it and it is still not sure to work. NOTE: This will drain your batteries much faster.
The fourth and final method is known as the RNG method, because of the Random Number Generator used in its creation.
First things first. Go to Lalivero and equip your party members with any items to ensure that Garet goes after Mia but before Fenrir and Ice Gargoyle. Probably around 150 agility, minimum. IMPORTANT! Make sure that you remove any poison, curses not counteracted by the Cleric Ring, and Hauntings on any of your characters. These would throw off the method. Now, go to the first room of the upper Venus Lighthouse (the one with the Psynergy Stone in the middle), save, and shut off your GBA. DO NOT SOFT RESET! You are now ready to begin this method. Turn your GBA back on and run around for a little while until you enter a battle with one Fenrir and one ICe Gargoyle.
THE BATTLE SHOULD GO EXACTLY AS FOLLOWS! The ONLY exception is the order of your guys. The only requirement is that Mia go before Garet.
First round: You attack first. If not, turn off your GBA and try it again. DO NOT SOFT RESET!
Ivan casts Tornado at both enemies.
Mia casts Ice Horn at both enemies.
Isaac casts Clay Spire at both enemies.
Garet casts Flare Wall at both enemies.
Second Round:
Ivan defends.
Mia casts Wish on all party members.
Isaac defends.
Garet unleashes Torch (Djinni) at Fenrir.
Fenrir Dies.
Ice Gargoyle does something.
Third Round:
Kill off the Ice Gargoyle. I'm not going into details, but it doesn't matter how you kill him. Take your time if you feel like it, it just doesn't matter.
Bonus:
If you use Bane to kill of the Gargoyle in the Final Round, you get a potion. I haven't used a RNG to test this, but it always works for me.
Hope this very long post helped.