Certainly Waterloo could have been won by a single tank if it went in on the French side. As long as the TC (Tank Commander) managed to keep from getting killed by a stray bullet, a flank attack along the reverse slope of the ridge along the Ohain road would simply have rolled up the British squares. The whole battlefield was only about 2 1/2 miles wide, so a sweep would have taken less than 1/2 hour. Then reverse direction and do it again. Using the main gun on horse-drawn artillery positions is real overkill, but a useful way to make noise, and a useful precaution against some cannon-cocker getting really lucky and damaging a tread.
Even if the tank ran out of ammo (and it would) it could simply run over any formation that tried to fight. In Patton's words, "grease for our treads". One of the first lessons out of WWIWWII was that flesh and blood, including cavalry, are simply useless against armored vehicles.
I'd guess the big trick would be to do it in such a way as to not panic the French as well.
Once you get to, roughly, the American Civil War, things start to get harder. Defenders were learning to dig in, and battlefields were getting much larger.