The ideal weapon is this:
Make exceptional bokutos then reforge with Ash kit until you get one with just -15 mage weapon. It CAN have Self Repair (as this will disappear upon imbuing) but NO OTHER PROPERTIES. Elemental damages do not count as properties, so that is OK!
Apply a whetstone to remove damage increase from the weapon. This makes room for the 15 DCI (since -15 mage instead of -20 mage takes up a significant number of points).
The final specs would then be:
Spell Channeling
(add Faster Casting 1 to negate the FC-1 from Spell Channeling)
15 Defense Chance
-15 Mage Weapon
If it's a bokuto and you have a 100% enhance tool, then you can enhance with bloodwood for 2 Hit Point Regen and 16 Life Leech (useless) or any other wood that you prefer.