Have a completed saved game file from Kingdom Under Fire: The Crusaders on your memory card or hard drive to unlock Gerald, Lucretia, Kendal, and Regnier in custom missions.
Successfully complete Walter or Leinhart's campaign to unlock Cirith.
Successfully complete Rupert or Cirith's campaign to unlock Morene.
Successfully complete Ellen's campaign to unlock Rupert.
Successfully complete Morene's campaign to unlock Urukubarr.
Successfully complete a campaign, then save after the credits finish to have the option to replay any mission in that campaign with your previous stats.
On the missions where you receive bonus experience for something (appears in green writing in the upper left corner), exit back to the world map and replay the level to keep the bonus experience. This is a good way to level up your characters early.
An easy way to kill Earth Golems is with cavalry. Make a waypoint on the Golem, then one away, and another one. Keep repeating this until the Earth Golem falls. It is recommended that this be done with Orc Riders because you can use Enrage to heal them. This is very useful in Urrukabar's first mission, as there are about five to seven Earth Golems to defeat.