playing anything above medium gives the A.I. enormous stat bonuses, to the point where a unit of line infantry won't be able to beat an enemy dismounted dragoons.

my advice is ot play with medium for battle dificulty, and campaign VH. that way you have all the same challenges, but battles make some sense.

as for upping xp, heres how i did it with marathas:

i my army was attacked by an anemy army, so i fought. i won, and the enemy was destroyed so i now only had to capture the militia-only guarded city. i for some reason only sent 3 units to fight. these 3 units were bargir musketeers that bore the brunt of the last battle, and now had a chevron each. the enemy sallied forth and attacked them, and i surprisingly own the battle. because the number of militia defending any city is for some strange reason 880 every time, and i had only around 3-400 men, they immediately went straight up to 3 chevrons each. basically, just take enough men that it adds up to about 500, and besiege a city. when the militia sally forth defeat them and you should have a bunch of xp. if you want you can make the city revolt and rinse and repeat until you are as high as you want. note though that you will probably have to merge units and revolve some units in and out of the combat zone to keep the XP.