Boulders can be rough, since they work on a percentage of the player's max health as a base number. The higher the star rating, the higher the percentage. When you add in elemental damage, it can roll out of control (pun intended).
Trap damage is still a work in progress, though. All of the battle numbers were recalibrated in the most recent release version, which includes trap damage. They'll be tweaked over the next few versions based on player feedback.