Nope. Even if the Reaver fired a third shot, it wouldn't be enough. The Ultra wins this. edit : Just realized I'm replying to a year old comment. Oops xD
A better matchup is Ultralisk Vs. Archon. In a First to 5, Ultralisk won 5-4, with 1 tie in-between (the tie didn't increase neither's score, so Ultra had a chance to survive of 50% and a chance to kill of 60%, for example). I think if I put them in a first to 100, the end result would've been way more precise, but I have no time to do so xD .