Heavy damagers do exactly what the name implies: they deliver heavy amounts of damage.
While it's easy to see that the Warrior Child of the Damned, with the highest ATK stat in the game as of the time of this writing, can deliver a lot of damage, others hide their ability by spreading it out over several hits or by scaling based on different stats. This is why it's important to look at the skills of each character to discover where their damage comes from.
A perfect example of hidden damage scaling is Fear of the Dark Eddie. His damage not only scales off of Max HP, but also off of DEF. These both seem like defensive only stats, but actually increase the damage each hit does. Essentially, the harder you make it for him to be hurt, the more likely he can hurt an opponent. It's an interesting effect for sure.
We'll take a look at each class and point out a few heavy hitters from each. Keep in mind, this list won't be all inclusive, but we'll attempt to get to the heart of identifying what makes each one hit hard and how to maximize their damage output.