I went with five castings. Four could have been expected to do the job (and did) but the way my luck has gone this game (I had to double check my scales just to make sure I didn't have turmoil/misfortune
) I figured since I had already spent 16 gems and four turns of mage time I might as well add one more casting as insurance. It was all very expensive but that that was also a tough target and wouldn't have been cheap to take out through normal normal attacks either.