Well, there's been some exlpination on fighting Bretts in this:
viewtopic.php?f=15&t=180
At 1.5, the only real difference in Bretts is that they can't take a Lord. Tactically, not much changes. They only get one stone thrower or Grail Knight lance. They still need one Knights of the Realm lance and outside of that, they're only really limited on points. The lance formation is their big thing and they usually go point, charge, slaughter, repeat. And beware the Grail Reliquae, as they're a tarpit you don't want to bog yourself down in.
Easiest way to deal with this is use skinks to divert the charges and annoy the hell out of the knights. Best tactic is bait a lance with skinks screening Krox, keeping the closest skink to the lance off to the side of the Krox. If they charge, they expose their flank to the Krox for a counter. If they don't, the skinks will continue to pepper them with javs or darts and the Krox may charge through the skinks to hit the knights if the distance is good.
If you're looking for magic, Beasts works well against them due to large number of cavalry. Metal also works wonders since most of their army sits at 2+ for armor.
And for the peasants: Leadership tests and fear effects should cause them to flee without too much hastle.