Do I get time to prepare battle field with mines and claymores?
I’m thinking XT is setting up the Americans with too many advantages :). How about a time/space travel swaperoo - the allied armies mysteriously sent forward and swaps places with, oh, let’s say the 1rst battalion, 504th parachute infantry regiment of the 82nd airborne out on maneuvers somewhere. The Americans wind up in Wellington’s general position with not a fucking clue where they are or what is happening - no pre-prepared ambushes, no idea what they are facing. Then Napoleon blunders in on them, mistakes them for a random allied enemy and starts to engage.
Then what?
Not quite they need two. I will tell you how this works.
Napoleon sees a US Army Ranger on a ridgeline of a hill. He sends a couple men to investigate. The Ranger disappears behind the hill as they approach and the sound of a scuffle is heard. The Ranger pops back up and doesn’t appear to have a scratch on him.
Annoyed Napoleon sends a company of men to go and retrieve the Ranger. Again the Ranger disappears behind the hill, and a scuffle is heard. Again the Ranger pops up and there isn’t a scratch on him and his uniform still appears to be nicely pressed.
Shocked Napoleon sends his entire Army over the hill to capture this man. A similar scene happens except this time great sounds of battle are heard. The sounds of men crying out and dying and the Ranger pops up again without a scratch on him.
So Napoleon is so Angry he marches towards the man determined to defeat him. However on his way up the hill he sees one of his men. The man is broken and bloodied, crawling on the ground towards Napoleon.
Mystified, Napoleon bends down and questions the man on what has happened.
Barely able to talk the man looks up and coughs out “Mon Empereur, do not go, it is a trap, There are two of them!”
This does strike me as more interesting. It seems obvious that a dense dug-in, prepared modern battalion could slaughter a frontal assault from an absurdly large Napoleonic force. But a modern unit wouldn’t bunch up their forces very much by default, would they? And if they’re spaced out, they might not be able to repel something like a cavalry charge.
If I were facing a charging regiment of heavy cavalry, I’d want (WAG) an entire platoon firing on them at the same time, because you’d only have 2-3 minutes to break the charge. If that platoon is spread out over a couple hundred meters, though, they might not be able to bring all their firepower to bear in time.
Well, I picked the Americans but that’s just me. As to advantages, of course…it’s war. Alls fair. At least I didn’t pick tactical nukes and mustard gas as some have. 
Plus I took away their vehicles.
In fairness, none of the stuff I picked is exactly exotic. The M-777 or even M-119 aren’t that uncommon, and a couple of batteries would be all you’d need, supported by maybe half a dozen 120 mm mortars.
And essentially I was thinking of something like the 1632 series where a small town in West Virginia gets swapped with a region in central Germany during the 30 years war. This time, it’s a heavy battalion (or whatever) of modern troops (from whatever country the poster wants) at their staging point but with vehicles en route to them gets swapped out with Wellingtons troops (who are pretty surprised to be in the future suddenly :p) and are hit by a French scouting detachment, and realize the French are positioning themselves for an attack in the morning. Penis ensues and all that.
I’m trying not to stack the deck too much, which is why I was asking for minimum numbers of troops. I still think a battalion is the minimum, as I think a company could be marched around and also wouldn’t pose enough of a threat to make the French engage. In reality (heh), if such a thing happened my WAG is that the modern guys would disengage or try and parley with the French, since they wouldn’t have a dog in that fight and there would be zero point to them duking it out with Napoleon unless the French forced them…which, I doubt they would, since what Napoleon really wanted was the engage the allied armies piecemeal and defeat them in detail so he could secure time for France to remobilize and secure his position back on the throne. He actually tried to negotiate first, but the allies weren’t having any of that, so he was pretty desperate to buy time. And here is a gift…Wellington’s army, at least, is out of the picture. That just meant he had to engage the Prussians, where ever they were…and, at a guess with Wellington completely out of the picture I’m not sure if the Prussians would be all that keen to engage a fresh French army.
But it’s fun to think about…at least for me. 