Difficulty affects several things, mostly cheats, or anti-cheats. I checked the XMLs real quick, and for instance, on easy the AI has the same build rates as you, has a mere 2x multiplier to their credit income, and only does about 70% of normal damage, whereas on normal, they do the same damage, build 20% faster than you, and get a 10x credit multiplier, which basically means they have unlimited credits unless you have beaten them completely into the ground. This effectively makes them more aggressive, since they can afford to build large forces more often, and by more often I mean they can literally do it constantly without caring at all about cost.
Autoresolve is very wonky. As far as I understand it, each unit has an autoresolve health value, and the autoresolve health total of the opposing forces is compared, and then some sort of witchcraft occurs, and then a result is generated from said witchcraft. The problem with autoresolve is that the autoresolve health values are totally arbitrary, so it's difficult to gauge how a battle will turn out unless you have overwhelming force, and even then you can get weird results like a single IPV-1 corvette somehow managing to destroy an opposing corvette despite the fact that it was alone against a fleet of multiple capital ships, corvettes, frigates, etc.
Personally, I would not recommend autoresolving land battles, at least not when you are on defense, since you will tend to lose, or take heavier casualties than make logical sense. On offense, go ahead if you want to, although make sure to bring overwhelming force and make sure that you can replace any losses quickly. The game never states it anywhere that I am aware of, even in vanilla, but if you build multiple structures of the same type on a planet, it will increase the build rate of any units produced by those structures proportionally.
If you're not autoresolving, my best advice is to try to gather as much information on the enemy forces as possible before going into battle, so that you will know what type of units to deploy first, and in what numbers. Learn the maps, and try to figure out where the AI likes to concentrate its forces, so that you can either avoid those areas, or hit them hard. Try to take landing zones quickly and land in force so that you don't get overwhelmed if the planet is well defended.
Also, in land combat you will tend to take at least a few casualties if the planet is well defended, use bombing runs and bombardments to destroy enemy strongpoints, some of the maps are very easily defensible, Carida is a really good example of this, if you try to go straight into the city portion with a head on assault, you are almost certainly going to take ridiculous casualties because of the sheer number of turbolasers and the dreaded capturable turbolasers (which for some reason have greater range and damage than normal turbolasers).