How are the Soothsayer DPS skills later on

[BB Shockwave]

Umm, the game still calculates damage the same way, IE +5% (I think?) after each point of intellect and +15% after every 7th point. Just hover the cursor over the intellect in your hero screen.

And the same way as before, you need to build up intellect via skills, shrines, items, etc.

Though even in the previous games, direct damage spells became useless after a while. A Mage/Soothsayer is better off using buffs and de-buffs plus various other spells to make his army stronger and weaken/disable the enemy. Not to mention summons.