Warfare in my campaign setting tends to focus on small, spread out units of highly trained professional soldiers. One of (but certainly not the only) reasons for this style of combat revolves around the sheer power of alchemical artillery ammunition, which is prevalent enough on the battlefield that any unit that marches in close ranks is going to get completely shredded, leading to a general military culture that sees spreading the troops out as essential, which in turn favors smaller forces of highly trained and disciplined professionals. Of course, if it is established that artillery is such a vital and dangerous thing on the battlefield, there need to be rules to back it up. I have Ultimate Campaign but haven't had time to flip through it. Do the mass combat rules there cover artillery well? Any good homebrew systems to look at? I'm looking for something that makes artillery powerful. Like, "put ten men in a line and an acid-spitting cannonball will come in and wreck them" powerful.
You should also look at the rules for Siege Engines in Ultimate Combat. The mass combat rules in Ultimate Campaign downplay artillery (it's more expensive and less effective than fielding cavalry, unless dealing with fortifications).