My battle plans are not very original since the AI isnt all that smart. A typical stack of mine is..

1 General
4 Howitzers
11-13 LI
2 Heavy Cavalry

I line all but 4 of my LI in a long line 3 rows deep. In the middle of that line and behind I line up my Howitzers. Behind the Howitzers I have 4 LI to guard against enemy cavalry attacks. If I use Cavalry, which most times I dont, I'll line them up either on the end flanks or in the rear to counter enemy cavalry attacks.

This is a very simple and basic formation that can beat anything the AI throws at me. The enemy AI usually launches cavalry attacks right away to my rear to get at my artillery. Once I have taken out the enemy cavalry I'm free to advance my LI and artillery forward without worrying about my cannons getting hit from behind.

In the beginning my Howitzers will pound the enemy artillery into submission and if I cannot achieve that then I send in one or two LI to kill the enemy artillery. I like to use a 3 row set up for my LI. Once you have Fire By Rank this set up works well because your units will fire three times. With the unit set up in 3 rows they can also deal with any charging enemy units in melee.

Like I said it is not very imaginative but its quite simple to execute and use and it'll deal with anything the game AI will throw at you.

In a fortress assault I use the same units, but no cavalry. I'll have 3 LI go around back of the fortress to assault there and 3 units on the left and 3 on the right side of the enemy fortress and assult the walls. The remaining LI and artillery attack the front. I'll let my Howitzers blow a hole in the front wall in one or maybe two places and then order all my LI to begin assault on the walls. Basically its an assault on all four sides. If the enemy has cavalry they will usually come out to attack my LI but putting them into squares will beat them off. Again its a simple battle plan but it does the trick every time.

Also on a side note in my campaigns I make sure to give all the other factions, major and minor, the same techs I have. The game AI doesnt tech up that well so by giving them all the abilities my units have adds a extra level of difficulty to the battles. I still beat them but it just takes a bit more time and effort on my part.