Your mage gets 1 slot immediately and next slots per 6 levels (at level 6, 12, etc.) You regain your kensai abilities at your kensai's level +1. If you do not want to waste proficiency slots, do the following:
1. Dual at level 9 - this gives you 7 slots.
2. Throw away the slot which you get immediately (assign it to dagger or so).
3. After reaching level 5, stop leveling up until your experience points allow you to enter level 10.
4. Level up (level 10). You will still be given the slot from level 6 but you also regain your kensai abilities. Therefore, you will assign the slot under kensai limitations. If you allowed entering level 6 as a mage, you would be doomed to assign the slot under mage limitations. |