I'll be honest, I view Drycha attempting to wade into the middle of the battlefield to reach Boris who's surrounded by two Ice Witches with Mathilde and Milica close by enough to intervene if they finish up with their respective opponenets to be a far safer environment than Mathilde confronting Drycha in the middle of the forest and expecting to win. Worst case scenario, Drycha kills Mathilde and moves on so she can capture Boris, and Mathilde, being dead, has nothing to contribute to the battle until she gets revived by the Seed.
Er... she is an Ulgu user, wading it not something she would have to do. Smoke and Mirrors too get to him, Mockery of Death to knock him off his feet and then another smoke and mirrors, probably combined with some kind of offensive battle magic on his guards to get out. Now would also be the best time to do it since everyone is busy and the reinforcements have not gotten here yet.
Now granted Boris could make his save against the spell... while fighting a Manticore, but I would not want to bet on that even accounting for the fact that she might not need him alive and an arm cut off with mind razor will do.