The half-elf Jerin had a pretty trivial victory over Tiamat last week, and, in spite of the theoretical higher level threat presented by her second opponent, this match should be as negligible as the fight against the dragon-like Fiend. After all, Mithos is basically as close as you get to a pure mage - and Jerin's Mirror spell is just the thing to entirely wall those types. Worse for Mithos, he's rather sluggish, which will leave Jerin (who is already quick) running circles around him. So, all she has to do is cast Mirror every now and then and leisurely snipe at the loser half-elf. There's no same race sympathy to be had here, only a one-sided fight in favor of Jerin, who'll get to show who's the more accomplished mage of the two.
