Humans are much more defensive than Orcs. The best way for a human player to win is to slowly wear down the enemy while fending off wave after wave of troops. This page will show you how to accomplish that attrition in the best ways possible.
In the early game, there is no difference in game terms between orc units and human units. So it is obivously to your advantage to strike before magic comes into play. If you deal the first blow, it could put you at a great advantage. So your first footman should be a scout. If you know the map you are playing well, scout out all start locations. If the map is new to you, remember that most maps have the start locations around the edges, and scout those out first. Try to find a pattern in the terrain and look at all places similar to the land around your town. Once you have found them, you should attack if you have the advantage. If you don't, try to sneak a few guys into his town and kill lumber peons. Why lumber? In the early game lumber is precious. If you kill his lumber peons, he will have to train new ones since it is diffucult to redirect a gold peon (due to lag). This will slow down his production greatly if he doesn't stop you. If he sends his troops to defend the peons, send the rest of your guys and attack his gold line. The object here is to make sure you take the advantage and hold it. If you get knights before he does, then you will have an easier time winning. Remember that these troops are kamikaze, and will most likely die (unless your opponent likes having dead peons), so you should be making more back at the base.
If you did your job well in the early game, you should have a tremendous speed advantage. After you get Knights and Rangers at level 4 or so, launch another attack. Use your remaining footmen first to soften them up, and then charge in with your knights and rangers and pummel them. If you did a good enough job in the early attacks, he may not be able to withstand this assault, and the game could be yours. However, often he will manage to fight you off (with peons if need be) and rebuild. Don't leave before you do some serious damage though. Blacksmiths, barracks, lumber mills, ogre mounds (if they even have it yet) are all prime targets. Even the great hall itself can be taken out (and this nearly guarantees victory). Never back down in the mid game.
When the game gets to castle stage, things change. Humans get mages and orcs get bloodlust. Tactics completely change as each side adapts to the strengths of the other. Mages can be used to destroy a lot of ogres with minimal losses to the human player. Here are some tactics to use:
The invisible mage trick is a method used to kill most or all of the enemy's peons. Most likely your opponent will not be expecting it, and his gold supply will be hampered severely (unless he's got more than one town, in which case your should bomb the biggest one.) First you will need two mages. Both need more than 200 mana (255 is best). They should have the spells Invisibility and Blizzard.
Cast invisibility on one mage and have him move into the space between the enemy town hall and gold mine. Then have him cast blizzard on himself (don't worry, it won't hurt him). Mosy likely his peasants will all die from the blizzard, as well as any troops that attack him. This is most useful if you also use demo squads to take out his barracks before, and attack right after.
This is probably the most feared and well-known of Mage techniques. Unlike the Invisible Mage, the Mage Bomb is designed to wipe out your opponent's troops. When done correctly, legions of Ogres (or enemy Knights) can be destroyed with nothing more than a mage or two. Here's how to do it:
Your mage should have full mana before doing this. First, place him by a group of ogres and cast blizzard on himself once.
Then, have him cast blizzard once over the enemy. (By 'once' I mean one shot of it, or 25 mana.) The troops will charge after the mage.
Finally, have him spend the rest of his mana casting blizzard on himself again. If it is done correctly, his troops will charge after your mage and into the blizzard. Don't try this if the enemy has catapults! You will almost certainly lose your mage in any case, but he will take a lot of ogres with him (or at least injure a lot). After this is done, attack! He will not have as many troops as you, and those that remain will be hurt.
There is also another technique, called the double mage bomb. In it you do the same thing, but a second mage (also with full mana) casts blizzard between the ogres and the first mage. If this is done correctly, both mages usually survive, and he has heavier losses than with the single mage bomb. (Very rarely, I also do a 'triple mage bomb', with a third mage adding to the power of the second. It is extremely effective but hard to pull off against someone who is paying attention. However the double usually is enough. Theoretically you could keep going, up through 6 or 7 mages, but they stop doing a lot of extra damage per mage when you get up around 4 or 5.)
The Footman Sacrifice is a way of killing two birds with one stone. It helps free up some food by killing your footmen while doing damage to the enemy. I won't illustrate it here because it is fairly simple and this page already has too many graphics.
First, get a mage or two and 6 or 7 footmen (if you only have 3 or 4, don't train more. Just use what you've got.) Have the footmen rush a group of ogres. While the ogres surround the footmen and pummel them, have your mages cast blizzard over the ogres. They will concentrate on the footmen, so the blizzard will kill most of them. You may lose a mage, depending on how far from your base this is done. (For extra insurance, have a third mage cast blizzard in the space between the ogres and the mages, like in the double mage bomb.) This will work with Paladins too, but you generally want to keep those in the later game, unlike footmen. However, if you want to kill more ogres, by all means send in some paladins. (On the other hand, if you want to kill ogres but have no footmen, you could do a mage bomb.) Again, like in all these tactics, attack right after it is complete. You have the advantage, but it may not be for long.