For farming, it’s mostly a numbers game and it’s pretty straightforward. Each troop type has a specific set of troop types that they are strong against, but you’ll want to diversify your army as much as possible between infantry, cavalry and ranged troops, and between each variety of those troops. Don’t forget about weaker troops even when you level up as despite being weaker, you can train a whole lot more of them on the cheap.
Always have some kind of construction going on. Pigeonhole any food, wood, and later on iron and mithril that you earn as quest rewards until you need it, because when you have yet to collect the quest rewards, it’s impossible for other players to steal them. When you do collect them, they then all of a sudden become fair game to other players.
If you get tired of attacking low level monsters but you want to do some easy farming, attack players who have quite obviously abandoned the game. You can tell because they will never level up and they will never seem to have any defense troops but they will have resources constantly building up.
Use your gold as wisely as possible. One of the best ways to use it is to hire extra builders for a temporary time. 250 gold will buy you a second builder for two days, which will double the speed as which you can construct things. This, while being temporary, has permanent effects that will directly affect the speed at which you gain power.
Use the college and the forge to research new tech and to unlock new weapons for your troops. Weapons can’t be directly equipped, but having them unlocked will buff your troops. Also, be sure to tap your commander and spend any skill points that you have earned before.