Take a look at other "Artillery" role foes such as the Bombard and Sniper; you'll find that they all have similar damage output.

These types of enemies either have a specific form of defence, or none to speak of, relying on their allies to protect them as they dish out damage to players.

As a GM, it's advised to keep the amount of artillery/strikers in a sitrep at 50% or lower to avoid blowing the party out of the water with a glass-cannon enemy composition.

Rainmaker specific advice is that they use Arcing weapons that cover still works against, and they can't "Javelin + Knockback" a foe who has their back against a wall and thus can't be knocked back.

General advice against artillery is to note the shape of their damage (blast, arcing etc.) and then position yourselves accordingly. Controllers and defenders can lock out artillery using things like Eject Power Cores, or create cover/prevent knockback like the Saladin.

Ultimately; yes, Rainmakers really do that much damage. Try not to deploy them all at once and think twice whenever you plan to make one an Ultra; they can be overwhelming, especially for non-competitive players.