Like I mentioned, Sectumsempra. It is not a "beam of light" spell, but rather just places it's effect directly upon the target. A spell like this would slash Sans all over, cutting through some of his smaller bones, and badly damaging his larger ones.
Protego counters all of Sans's attacks, and Voldemort has been shown to cast a powerful enough Protego to resist Dumbledore's attacks. Plus, Voldemort can keep up such a spell indefinatly. Although he wouldn't be quick to cast the spell due to his arrogance, he would be very quick to once he sees the million bones and Gaster Blasters pointed in his general direction.
Fiendfyre is a heat-seeking missle. Sans can teleport whereever he wants, Fiendfyre is following him untill he finds a way to destroy it, or it destroys him.
Voldemort also possesses teleportation via Apperation/Dispperation. He can teleport tens of kilometers with it, far out of range of Sans's most powerful attacks. His wand attacks travel hundreds of meters, far further than Sans's bone attacks, which go dozens of meters max.
Voldemort can also keep this battle going for longer, he has never been shown to tire from spells, even when casting the most powerful Unforgiveable Curses, whereas Sans tires after 5-10 minutes of hard battle.
Sure, in terms of battle stratagy, Sans has Voldemort beat, but this doesn't really matter when Voldemort has so many spells that can make Sans's life very painful. Voldemort can throw spells like nobody's business, and has even been shown in The Order of the Phoenix to cast spells even while behind a Protego charm. Even if he isn't smart enough to use Sectumsempra, or Fiendfyre, he can still use the "spray and pray" tactic, forcing Sans into perpetual dodging, with not enough time to get an attack off himself. And, Sans is a glass cannon, one good hit with a powerful spell should be enough to put him down for good.
