1) Find an easy mark. Such as someone who puts his keep out, this way, you can win and gain crowns with as little troops as possible.
2) Use your heroes to your advantage. Heroes are more than infantry or cav with high health and damage. They have abilities that can alll help turn a tide in battle, for instance:
Richard can be paired with Salza to make foot units a very dangerous tide, especially if they are ladder invaders, as ladder invaders have high health, attack, speed and climb over walls. With the speed, attack, and defense bonus of Hermann and Richard, they can take out a good number of buildings.
Or use Charles to distract defenses while you destroy a keep.
Or Edward to win in a matter if seconds with conrad and some siege weapons.
Or some other great combo. (Belisarius + Edward, Nikephokas + trebs+ Conrad, Maslama + Saladin)
3) No man gets left behind. Defend and re-defend. There is no use in spawing a treb. battery if you won't protect them. Using this logic, use bunchged troops to easily get through, as they will do lots of damage and won't get in eachother's way. But beware of catapults, fire traps, crossbow towers, flame towers, and balistas using this tactic, as they can all hit multiple units at once.
4) Range. Range is a key figure. Use trebs or other high-ranged units (such as archers, longbowmen, or onagers) to shoot over walls or out of the range of towers, this way, you can damage them, but they can't damage you.
5) Demolishing groups. Attacking groups can go both ways. Onagers, longbowmen and some heroes (Winrich, Alexander, and Tariq) can attack multiple units or buildings at once.
6) Scouting. Scouting can help you tons, that way, your cav rush doesn't run into a fire trap, or so that your ladder invaders don't get stuck on caltrops and become sitting ducks. Do this by using infantry -or another minor unit that won't cost you much- and literally just running around.
7) Organization. Don't send all you troops in one big bunch, use them in a good order, like infantry, then siege weapons, then your long-ranged not-so-durable troops.