This could go either way. Malak is generally considered to be about even with Dooku, perhaps slightly stronger (most people thought Malak would beat Sidious in the Malak vs. Sidious thread, actually). So Malak will knock Obi-Wan out like Dooku did and then it'll be him vs. Anakin. But I think Malak could use Force Darin on Obi-Wan to increase his power just as he used Force Drain during his final duel with Revan and therefor would not get tired as quickly as Anakin would and he'd managed to finish off Anakin. But that's a real close battle there.