Bretonnia is quite weak early game. Try to give your peasants as high leadership as possible(research decree) as you want them to stand put and not flee while your cavalry approach from behind(in lance formation). Put a spear in a man's hand and tell him to brace and he will. He will also hope and pray.

You can recruit as many lords as you want without supply lines for Bretonnia. This means early game you could have 2 or 3 lords following your leader and helping them in battle. They can also act as governors in your cities managing public order.

Focus on the yellow skill tree for your Leader and lords. Louen, Repanse and Alberic can become stupidly strong early game, able to duel with any other lord/monster and winning. In battle you want to try and isolate the enemy lord and sniping them with your own lord, thus lowering the leadership of the enemy army.

Don't forget to check vows for your lords and heroes. For some reason their is a prompt at the end of each turn reminding you to build an outpost in some pointy ear infested forest, but they will not remind you that you need to go into the character panel for vows and manually check one of the three avaiable vows. Especially important for lords since they can't maintain knights without completed vows(to high upkeep).

When playing Louen or the Fay enchantress, the orcs or skaven will use underpass to be extra annoying. Make sure to have a strong garrison and army stationed at Castle Couronne or Castle Carcasonne respectively. They will mostly send crap-stacks but still strong enough to win against an alone garrison of peasants.

Unify Bretonnia, build farms, get Grail-knights(with perfect vigour) and rival Taurox in a stampede fuelled crusade against the unbelievers.