So basically your plan is to strap on 40 pounds of burlap, put down your modern weapons and start a knife fight in the dark with a bunch of armored knights?
tempting; but you do know don’t you that like antibiotics, army cooks only kill the weakest, leaving the most deadly to respawn? 
Too close; the point of the thread is to engage them while they’re still at least 500 years away. 
The really stupi thing is that I had intended to write “meters”.
Okay, so then what’s the lowest technological point at which a single modern unit can dominate that battlefield?
Let’s say I show up with a single, fully armed and manned M1A2 Abrams. Is the opposing army pretty well fucked now (at least until we run out of ammo and/or fuel)? What about just an armored Humvee with a .50 cal?
Any WW2-era MBT would totally destroy any enemy of that era. Button up, use the co-ax to take out groups and the main gun to pot the generals in the back.
Those temporal rounds can really ruin your day.
I think a modern backpack flamethower would be pretty effective. Showy, scary weapon that wouldn’t be considered witchcraft, since it’s just an improved version of Greek Fire. Fill some trenches with pitch ahead of time and set them alight with the flame thrower for added effect.
Shouldn’t be too hard to reverse engineer either, so you could make more, as long as you knew the chemistry.
Arrows, on the other hand…I’ll need to think on that.
Could someone please create this video game? Thank you.
They pretty much did this in the movie. The use of fire as a weapon was well known. The range of a flame thrower is limited, and it is most effective against an enemy hiding in a cave.
Somewhat relevant links starting with speculation on Reddit of how a Marine Expeditionary Unit would fare against the Roman Army. Looks like a movie is in the works…
http://www.reddit.com/r/AskReddit/comments/k067x/could_i_destroy_the_entire_roman_empire_during/
http://en.wikipedia.org/wiki/Rome_Sweet_Rome
Claymore mines?
Wouldnt take many of those in a set battlefield to really ruin the day for one side.
Otara
They did. I played the demo and it kind of sucked. It starts with time travelers rescuing a soldier from Custers last stand. The part of the demo I played had you fighting in the Civil War with a submachine gun. Neat concept that seemed poorly executed.
You wouldn’t even need a sniper. Any soldier who passed the basic proficiency requirements for a rifle would be able to pick off the enemy’s leaders with little difficulty.
I can’t fault your tactical judgment - but damn, man, what did those horses ever do to you? It’s not like they want to be there. They just want some nice grass, and maybe an apple or two. 
I think that’s the way they did it with the pikes though (not just from the movie, an actual battle tactic). It’s a lot easier to target a horse, and when they go down they create an obstacle, and are more likely to spook the other horses than knocking the armored guy off his mount.
In particular, the enemy quickly realize that they’re being shot at by someone intent on taking out their leadership in particular, and quickly take what seems to them to be the appropriate action: To scour the woods on either side of the road to find the skirmishers hiding in the underbrush, because skirmishers in the woods is the only logical explanation. But of course, the sniper is actually hundreds of meters away on a hilltop, and they don’t even come close to finding her, since it never even occurred to them that it was possible for a shooter to be that distant.
In a way, going back to the pre-firearm era might actually decrease the impact of the modern soldier: An enemy who’s used to early firearms will already have very strong notions of what’s going on, and those notions will be wrong. A pre-firearm enemy, though, will realize that they don’t know what’s going on, just that someone is killing them with something like thunder. Without knowing just how thunder-weapons work, they just might figure out that it’s coming from that person up on the hill, and charge there.
Oh, and I know that the OP specified ordinary military equipment, not a sniper rifle, but compared to the weapons of the time, an M16 is a sniper rifle: It’s long-ranged enough and accurate enough that you could isolate individual targets from further away than the enemy can effectively retaliate.
Heh, I kinda did something like this with Bungie’s old Myth 2—someone had, with the generous modding tools available with the game, made a Vietnam-era total conversion mod. Mostly for online multiplayer, but you could play a single-player version of the game’s default campaign, with the modern units replacing the medieval/fantasy units.
Tinkering around with my copy, I hacked it to replace all the hero units with the USMC guys, but leaving the villains (mostly various flavors of undead, with some monsters) the same.
You don’t know fun until you get to call in an Arc Light (with that same movie audio clip) against a horde of skeleton archers trying to force a bridge crossing. 
Well the OP was a two-parter, first if a regular modern western soldier with standard equipment was dropped into the battle and the second bonus fun ‘bring your own’ part where the soldier gets to bring what he wants as long as he can carry it. 
It’s been a long time since I’ve seen the movie, whatever battle he was at then.
Thanks for the answers everyone!
No. No matter how much more advanced your weaponry is, you’re still only one guy against a whole army, and therefore must employ asymmetrical warfare. You should hide until almost everyone is asleep, and do something similar to what the Vietcong and Japanese did to freak out American troops. Prowl along the edge of camp with your night vision goggles, find two sad sack peasants (impressed into service by their lord) sleeping side by side. Cut the throat of one of them. Repeat this a dozen times, stealthily, then creep back to your hiding place. Do something to wake everyone up – maybe a flare? And a dozen guys wake up to find their close companion dead. The peasants start screaming and running around, and the knights run out of their tents to shout at them, strap on their armor and call for their horses. And at this point start popping off the leaders with your night-scoped rifle. The whole army will be running back home before daybreak, convinced that the devil is after them.
My solution is much more elegant then twenty hysterical minutes mowing down horses with a machine gun before you’re finally overrun by halberdmen – wouldn’t you agree?