Outriders

Outriders is a Empire cavalry unit in Total War: Warhammer. Fast and dependable, an Outrider can be in one place to fire, then be in another to fire again moments later.

Description
Outriders are grim men who are both the commanders and drill instructors of the Pistolkorps. Outriders are rarely noblemen, but rather they are veterans elevated from the ranks of the state regiments, sergeants and officers who have shown an affinity for horsemanship and a talent for training new recruits. Outriders typically wear ornate armour, for noblemen pay well for their sons to learn the art of war. They also tend to sport eccentrically waxed mustaches to differentiate themselves from their younger charges. Outriders also form into separate regiments, sometimes to show their charges exactly how it's done, but also to display their own martial discipline to any noblemen in the army that might be looking for an experienced instructor to tutor their own sons. As the Pistolkorps is funded in part by the Imperial Engineers School, Outriders are equipped with deadly repeater handguns, weapons capable of unleashing devastating storms of lead at long range. The leader of the Outriders, inevitably the keenest shot in the regiment, will often be armed with an even more outlandish weapon as befits his status, gifted to him by a patron engineer.

Strategy
Outriders (All instances being discussed are the standard repeater rifle variety) are in fact one of the best units in the game, particularly during the campaign. Being statistically identical to handgunners, other than trading some unit size for mounted speed, for 25 more gold per turn, you turn your unit of handgunners into a mobile, quick-strike task force that when used even to a quarter of their full effectiveness will turn the battle into a much easier endeavor for you and your troops. Important to note however is that their pathing AI will not put the entire regiment in range to fire on a unit for some reason. So using move orders to place them inside their normal 135 range can help get the maximum firing damage. Its worth noting however what you're paying for here is speed, firefighting capability and missile piercing harassment. Of course they aren't as good as handgunners at stand and fire tactics, but if they were, handgunners would be 100% obsolete. As a general theory in this game, comparing units head to head is not a good idea. Comparing units to situations is.

I personally recommend in every army you take 1 to 3 of your ranged units and replace them with Outriders. They shouldn't replace all of your ranged units, but having just a few of them around gives you incredible bonuses. The basic tactics to maximize effectiveness from these wonderful troops are:

Harassment: You can pitch them out before the battle to get off a few shots at some high end troops the enemy has, and outrun just about everything to get back to your lines before the fight. Consider it precision artillery during that opening phase of the battle. Taking 2 units of outriders with 2 units of pistoliers is a perfect way to dismantle enemy cavalry, especially if you use banners to boost your outrider speed. The pistoliers can constantly harry the enemy until they have to respond or be shot to pieces. Keep your outriders at a fair distance away to lay shots into the responding cavalry. If they turn to go after your outriders, keep distance with the unit they're chasing and firing with the unit they're not chasing. The pistoliers will continue to fire into them as they're being chased, and can also effectively be used to charge down enemy missile troops or artillery pieces while the outriders mow down the heavily armored cavalry responding.

Firefighting: Their mobility combines with their awesome armor piercing firepower to create a force that will be able to go around and crush the push the enemy was depending on to win the battle. Just get a unit of foot troops stuck in against the enemies best, heavily armored damage dealer. Circle around back with the outriders, open fire, a few seconds later the battle is now in your favor and their unit is either dead or running broken from the effects of being flanked and raked by missiles. Firing from the back reduces enemy armor, gives them no chance to block, and is a huge leadership reduction. Because of the mobility of outriders combined with the deadliness of the same weapon as handgunners, they can perform this task to an incredible degree.

Anti-Large: Combine very effectively with halberds to kill those single enemy per unit monstrosities like arachnaroks, giants, steam tanks, etc. because they can fire their deadly armor piercing rounds freely over the heads of the troops pinning the monster down. While this can be done by hand gunners, it is important to note how you risk losing an entire unit in the time it takes to march the gunners into position to do so.

Chargers: Unlike Pistoliers, if you give a unit of Outriders an open back or side of a unit and a charge command, they will inflict a good bit of damage on the enemy without dying foolishly. You should use this to make units waver, pull pressure off certain foot soldiers, or to buy time for the heavier chargers to arrive. Simply pull them back after the charge and begin shooting again and you're making excellent use of your elite light cavalry.

Capturing enemies: They are faster than your standard heavy cav, giving them excellent chances to capture enemies that break after the battle. This means more gold, replenishment or leadership in the end. Can definitely be useful because of their attacks reducing leadership, meaning routing units trying to rally will take off again.

Basically, if you aren't using at least a unit of Outriders per army, you are automatically lowering your potential as a general. Outriders perform excellently versus each faction:

Empire
Easily can turn the tide of a battle by letting enemies fall into a firing line trap. Let the knights charge, circle around and shoot them. Bog down expensive units like armored knights and greatswords with your cheap tier one alternatives, circle around them and shoot them in the back. Even the dreaded steam tank is easily smashed by a few volleys. Its easy to take out units of reiksguard and demigryphs with two units of Outriders, one shoots and feigns retreat while the other follows behind shooting them until they turn around.

Vampire Counts
Terrorgheists, Varghulfs, Vampire lords, Crypt Horrors are all favorite prey of the outriders.

Dwarfs
The only thing outriders become a liability about here is that they eventually run out of ammo, because they are terrifically effective against the heavily armored halfpints. They also tend to leave their artillery in places where the Outriders can easily disable it.

Chaos
Super slow, heavily armored guys. Do the math on your own.

TL;DR- PEW PEW AT HEAVY ARMOR AND BIG THINGS WITH HORSE GUNNERS AND A WINNER IS YOU