Here is a post for those people who complain about spam and such, say cavalry is pointless, or just complain in general. I present: The Balanceogram! Of course everything CAN kill everything, but to demonstrate the validity of every unit I have made this sexy diagram.
Skirmishers: When used properly skirmishers can be effective against anything, especially if you use long rifles or w. jaegers. Stakes can protect flanks, artillery, or whatever you don't want to have charged, but the best way to use skirmishers is against lines of infantry. Because of their range you can get the first volley off and then retreat to a safe distance. Since they can go in loose formation, your line inf behind can also fire through the unit with minimal friendly-fire casualties. Place skirmishers on the flanks to demoralize line inf while firing or in melee engagements for optimal results.
Line Infantry: The standard units that everyone brings to the battlefield. Downsides include poor agility and short range, while benefits include bulk numbers and square formation. Line infantry is generally used to beat...other line infantry, but for balance purposes they have been specifically tailored to repel cavalry via the square. If you see an unprotected charge coming on a flank jump into square to buy yourself some time. That unit will become bogged down and quickly lose moral from flanks being exposed.
Cavalry: When I heard people putting a limit on skirmishers I was floored, or banning the use of long rifles and w. jaegers. The simple solution to beating skirmishers is cavalry. Yes skirmishers can put stakes down, and yes sometimes their stealthing ability can cause cav to be surprised, but if you keep any eye on skirmishers you will notice where they are. If you keep your ear to the ground you will be able to track long rifles skulking through the grass. For most skirmishers (even elite ones) all it takes is a swift charge to the side or back to rout. I usually use light dragoons who have a piss poor charge, but still do the trick. Generally speaking cavalry can be used to flank line infantry as well, but normally those pesky skirmishers have to be dealt with before this is a possibility. Lastly, they can be used as a quick method for riddening the artillery menace of your enemy, especially if they abandon their position or have invested too heavily in art. If anything, heavy cav is unbalanced due to fatigue, so use them sparingly if you use them at all.
Artillery: Universal killing machines. Generally speaking they do not do too much against skirmishers because of loose formation, but can kill over a dozen men with a single percussion hit even while loose. Art kills everything and when properly protected (with its infinite ammo) can cause numerous tactical problems for your opponent.