There is no best formation for heroic realm in general. You need to adapt to each situation, which requires some thinking. If you are having trouble with a specific boss, I am sure people will be glad to help you work it out. For my part, switching my formation around has usually been enough. I rarely have to use different heroes.

Here is some food for thought you may want to consider when experiencing trouble:

- Is there a weakness in the enemy's formation? How could I use it to my advantage?
ex: Venomius in the storyline (not a heroic boss but a good example) has a lot of hp but does not deal a lot of damage. Having Maria will allow your team to last the battle and thus defeat him.

- Is there a strength in my enemy's formation that I need to minimize? How?
ex: working on your formation is helpful here in avoiding some skill attacks (piercing thrust/sweeping slash) AND in maximizing the use of your own skill attacks. If your enemy has thousand strikes, you might want to use calvin, for instance, to prevent him from using it.

- Are there heroes that could help you in performaing better within a given situation?
ex: Cianna, Rhize and Agripus can hit the back row and can be helpful in dealing a lot of damage quickly with their skill attack. Ahmose will be useful against physical attackers, but not that much against mage bosses who use Horns of Hell.