3 dice with a level 1 skink priest at a casting value of 10 is about 74% chance of success. Throw in another dice, at it lands at about 95% chance - but the miscast chance also goes from 7 to 13. If you bumb his level up to 2, you suddenly have a 83% chance to cast. Adding another dice would be pretty pointless, unless you really need that wyssans, or have nothing else to spend your power dice on.
Then make sure to cast wyssan's last. I'm not taking a 17% chance that my magic phase is going to be over.