Kind of simple, but good
I have a strategy that generally wins. Basically, you use halberds and spears, all in the same lane. I am doing it with the Orcs for the +2 halberd. I had previously won the game using this strategy with Night Elves (who have no advantage for this strategy).
The Halberds are your Tank. You send out a good head of Halberds first (all on the same lane). The number of Halberds you should send depends on your enemy. If the enemy is fairly strong (like the Demons) you might want to send out 10 Halberds before sending the spears.
The Spears are the Fuel or Armor for your Tank. Halberds on their own will get pushed back but if they have spears next to them they push forward. Spears on their own will get destroyed (if the enemy is strong) but if they have Halberds next to them they are more powerful. Once you have your first 5-10 Halberds out on the field, send out a steady stream of Spears (in the same lane as the Halberds, of course) to support them.
The goal is always to get your units into a clump. A clump of 5 Halberds very close together is virtually unstoppable when backed by spears, and a clump of spears is nearly as strong. A lone enemy, even something like a horseman or a flailer, doesn't stand a chance against a good clump of Halberds and Spears. Don't complain if you get pushed back by an axeman or horseman since that only makes your units clump up more!
You want to push your Tank (your clump of Halberds) all the way to the enemy side, by continually feeding it Spears. When you are done with that (or nearly done) it's time to send out another Tank (clump of Halberds). Rinse and repeat.
Other advice:
-there's an option to spawn continuously. Obviously, use it!
-Upgrades: upgrade your Speed to 2 or 3. If your units are faster they will clump up more on the enemies. Also upgrade Spear, Halberd, and Armor.
-I like to save my rush of 8 units for just after the enemy has done his rush, so that I can send a bunch of Spears over the finish line unopposed.
-Don't try to stop the enemy from sending units across in other lanes. Just let them go, it just means you have less opposition in the lane you are attacking with.
-Use your common sense. If the enemy has formed an unstoppable clump of strong units in your attacking lane, don't waste time trying to defeat it. Just switch to a different attacking lane.
-Try to time your first wave of spears so that they arrive at the front of your halberds just when the first strong enemy gets there, so they can join in to defend.
-The way you get money is by kills, not by winning battles. If you want to get more cash you should try to make a battle drag on endlessly so you get a lot of kills.
-Squish the strongest enemies first on the map (by percentage). If you leave them alone they will get too powerful. It's OK to leave them alone for a turn or two, but the longer you wait the harder it will eventually be.