the only way for voldemort to win this one is to hit yoda with AK or cruciatus before yoda gets going. essentially, to take yoda by surprise. however, yoda has legitimate precognitive abilities, plus the ability to read auras/minds, so he will know well in advance that he is fighting a dark force, someone with no mercy, who can kill with a word. yoda isn't getting taken by surprise.

we've seen that voldemort's spells are dodgeable, and yoda's dodge feats are the best. even if voldemort takes yoda somehow by surprise, he can't get a hit in.

given preparation and some strategic assistance from someone familiar with jedi powers (like dooku or sidious) voldemort might be able to put together a win. HP magic is extremely versatile, and we never get to see half of what voldemort is really capable of (or so it's implied). but that just isn't the way voldemort behaves. he isn't strategic, he's arrogant. he isn't a team player, and he doesn't take advice. he relies on terror a lot of the time, and yoda just isn't someone he can intimidate (HP illusions/mind control vs. jedi mind-tricks are a wash in my opinion, with neither combatant really being able to affect the other).

there is one conceivable way voldemort takes this, and that's if he does what Dooku does when he fights yoda: he endangers bystanders so that yoda has to concentrate on saving lives. yoda would sacrifice the win, and perhaps his own life, to save the thousands of dwarfs in the arena from falling rocks, etc.

however, given the presence of popeye (a superhumanly strong good-aligned human whose feats of strength have basically no upper limit; like the hulk, but somewhat more intelligent), if voldemort starts ripping down the coliseum to kill dwarfs, popeye will join the fight if only to save his own people.

i'd say yoda takes the fight to the death 9/10 times. if voldemort is trying to escape, however, and uses his powers to kill bystanders, yoda lets him get away 10/10 times.