In the Crusades event, you face 15 progressively harder opponents. The opponents are clones of players on your server, except the battle rankings have been adjusted. As you go further, the BR of the opponent increases.
As your main character increases in level, you get more HP. However, the opponents also get harder. There are many trade-offs to leveling vs. not leveling in Shadowbound. Staying at a lower level will help you go further in the Crusades.