The fight really wasn't over.
White was still alive and the Holy Grail was still inside the stomach of that large forest boa. Marvin couldn't leave.
He had planned on getting rid of both White and his guardian earlier.
But White's abilities exceeded Marvin's predictions.
Not all wizards were wastes that only knew how to study spells. Obviously, White was able to reach the level of having real aptitude.
He displayed very impressive abilities in front of the large forest boa. If not for that accurate Ray of Frost, he might have already died.
Indeed, Marvin originally wanted to kill the wizard while they were fighting the boa, making it easier to kill the guardian.
But the large forest boa lost its sense of direction after getting hit, forcing Marvin to find another way.
So he just killed one first, and went on from there.
It was now a one versus one.
