This is easy, occasionally while wandering the map you will see lords battling bandits. There's two crossed swords and the words BATTLE when you hover over it so you literally can't miss it. Just jump into the fight before it ends and win. If you are in a faction during wartime the Marshall will usually call you to follow them. During this time they will almost always get into a battle. Just make sure you aren't the starting party and jump in while the battles going on and of course win.