Empire: Total War

To "flank"

When possible have your men fire into your enemy's flank. I.e. have your men cross the T. Of course your enemy won't let you get away with it easily but that is what you want to achieve. Flanking them in the back is the most destructive of all if you can get into that position, but that's generally harder to do than crossing the T.

To start a rout

Your objective in the battle isn't necessarily to kill the other guy, but rather it is to rout them and cause them to flee the battle field. A method of doing this is to put heavy pressure at 1 point of your enemy's lines. A well placed canister shot or explosive shot can hit your enemy's moral very bad, use them. When they start to waver (flashing flag), sometimes all it takes is a massive charge by your line or your cavalry and it will start a chain reaction of putting the rest of your enemy's line in a panic.

Just be careful about Friendly fire with the field guns. You want them either on a hill just behind your line, or just in front of your line. The former is the best position, while the ladder your gunners will probably take hits when the musket balls starts flying but it may be the only choice on a flat battlefield.

Crossing Ts work on land or on the ocean. It's about as fundamental of a tactic as you can get.

Units aren't necessarily "better" than others, it's more of what they are good for and how do you exploit their best abilities. Line infantry is pretty much your standard fighting unit. They are good at holding ground and absorbing damage to a point, they can be deadly against cavalry if they are in a square formation. But the trade off of a square formation is that you make yourself more vulnerable to artillery.

Artillery lets you hit your enemy way before they can hit you back and cause a relatively large moral hit. Solid shots at the beginning of the game are pretty much only good for hitting cavalry at a distance, but once you research to the bottom of the artillery tech trees, the shrapnel shots and percussion shots are extremely deadly at a distance. I have had instances where I eliminated 1/4 of my enemy before my lines were engaged. However Artillery is only as good as where they are placed. Also if your guns are unprotected and are at melee range against cavalry or infantry, you can pretty much kiss them good bye, don't let your enemy get close to your guns without protection from either infantry or cavalry units.

Cavalry is the mobile arm of your army. Their advantage is being able to move around the battlefield fast. You can use them as reserves and help a part of your line that may be in trouble or have them threaten your enemy's artillery. They are most effective when they charge at wavering enemy and causing a rout.

Another general tip. Always keep a few units in reserve (depends on the size of your army of course), so you can either plug holes in your line or exploit a gap opening up in your enemy's line. Don't commit everything you have at the beginning because if something goes wrong and you have no reserves, it is all over for you.