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Bretonnians vs. Skaven
The Skaven are a mass army, and must be dealt with more carefully than other armies. The biggest strength they have is in numbers, as it gives them rank bonuses and a high leadership. The only easy way to overcome this is to flank the large units. In terms of missile troops, there is very little to worry about. As with all war machines, they are deadly to units of knights, as most hit with a high strength or negate armour saves altogether. (The warpfire thrower is the perfect example.) Skaven magic is also very devastating and has to be taken out early to avoid any possible disasters.
Flanking large units can be meddlesome, but it is relatively easy. The key unit here is mounted squires. Don't arm them with anything; their sole purpose on the battlefield is to flank the massed ranks of Skaven, so that they lose their leadership AND rank bonus. Once the unit is flanked, any charging unit of knights will summarily hack the rats to bits and leave very little chance for them to run. Biggest problem here is if the damn rat's got censerbearers surrounding the unit. Another no-no for the knights. Since censer bearers are frenzied, they have to charge a unit that is in charge range. Send a small unit of skirmished mounted squires ahead to draw the censer bearers away. Once that's happened, flank the unit as planned and crush them with a unit of knights.
The only thing that can stop the knights from breaking the unit is the crown of command. The answer to this is to send a Bretonnian character to kill the vermin that's wearing it. It's the only way.
A hero mounted on a pegasus is a good choice to have to hunt down the skaven mages. They tend to be tough, so give your hero an offensive boost to counter this. You won't need the virtue of devotion. Have a mage carry a dispel scroll or two to counter serious spells.
The war machines of the Skaven can be dangerous if they work for them. Use lots of bowmen to disable these units.