Raiding is the art of taking as many provinces as possible with as little effort as necessary. A raiding group is the opposite of a doom stack. Raiding does well against unforted provinces and can do a great deal to an enemies' gold and gem income because they can't collect taxes and gems from provinces that they no longer own.
A raiding group can be anything from 40 zotz who routinely suffer 50% casualties; to 3 anakites and an indy priest; to a mage with a few gems set to drop two elementals and take a nap; to a thug. Quality is binary, either you take the province or you don't, so raiding is all about quantity. Effective raiding tactics involve a variety of raiding groups because you must use all resources available.
Raiding is successful when you can raid more provinces than your opponent can take back. This is known as applying lots of raiding pressure, pushing the front line back to the enemy and leaving you with more and more provinces every turn.
Both raiding and defending against raiding are games of tradeoffs. The smaller the pieces you split your force into, the more land you can take (as the raider) or defend (as the defender). But, conversely, the more you split your force up, the less likely each individual piece is to win its battle. The more value you commit to any one province, the more likely you are to win that battle – but, often, the more you stand to lose if your opponent commits something that counters you there.
So, ideally, you want to split your force into as many chunks as possible that are still likely to win their battles – and commit your valuable resources so that they have maximum impact but minimum risk.
A couple of things to think about:
* How can I keep my opponent off balance so I attack into minimum defenses? Raiding forces, by definition, are small enough that your opponent could kill them if they fought them with a large force. This means you have to strike where those large forces aren't; this means that you are more likely to win your battles. Raiding requires good scouting: you need to know where those large forces are and where they could move to intercept you, and allocate raiders so that you avoid the defenders you can't kill, but plan raiding parties that can kill anything that your opponent could get there to defend with. Mobility is extremely valuable since it means you have a greater number of provinces you could possibly attack, making it harder for an opponent to intercept. Stealth is very good on both sides, since the attacker can use it to hide their possible attacks from the defender and to infiltrate; it is also good for the defender, since if the attacker can't see the defending forces, they can't plan for what their raiding parties might need to fight.
* Can I kill whatever my opponent has there to defend with?" The lightest raiding squads are designed to kill 6-15 PD. If you are raiding into PD, a small force – a few Ulmish black knights – will suffice. But there are harder targets to raid too – such as blood farms. If you're attacking a blood hunting operation, you'll have to fight patrollers and hunters too. Make sure you plan for this.
* If my opponent does commit defenders to the province I go to, can I kill them? Your opponent will likely split their force to try to deal with your raiding forces. Normally you hope not to encounter defenders other than PD. But if you can manage to commit something else to kill possible enemy defenders, so much the better. This can mean thugs, fluffers or light mage support, or a mix of raiding unit types. Obviously your raiders can't fight the whole enemy army, but if you can make your raiders able to win small skirmishes, so much the better – it forces your enemy to keep their army in bigger groups and makes it easier for you to keep them off balance.
* What is the risk/reward of this attack? Often raiders are not dedicated raiding parties, but groups of troops or thugs that get "drafted" into raiding duty as opportunities arise. Sometimes you have chassis that are extremely valuable, out of proportion to what you might gain with a successful raid. For instance, an Air Queen is usually an army support caster. But with 20 gems of gear she can make a useful thug that can capture most provinces. But she will likely die if she hits an enemy army. 70 gems of thug is not worth risking over a humdrum province. Does this mean you shouldn't use air queens to raid? Not at all – but you should use them safely.
* How can I mitigate disaster? Flying is a very useful trait to have on a raiding thug – because they'll manage to retreat from battle more often if things go wrong. When raiding, it's best to attack groups of adjacent provinces – that way if something needs to retreat it has somewhere to go.
* What will the counterattack look like? Raiders without stealth can be attacked in the magic phase on the following turn. Could you lose your raiding force to a teleport/cloud-trapeze counterattack? (And do you care? If it's five Ulmish Black Knights, probably not – but if it's an air queen, you absolutely do!) Are your commanders liable to get hit with Mind Hunt or remote assassinations?