Attacks that hit automatically must still roll to determine if they hit Soft or Hard. Hits that wound automatically must still roll to damage the Tank as normal using the Strength of the creature delivering the hits.
For example, let's consider the Thump with Club attack of a Giant. The attack hits automatically (the Tank failing its Initiative test automatically), but the Giant still has to roll to see if it hits Soft or Hard. Let's imagine it hits Soft. The Giant's club will therefore cause: D6 + 6 (his Strength) + 2D6 (the multiple wounds it causes) - 8 points of structural damage against the Tank An 'Eadbutt attack (one automatic wound) hitting Hard on the other hand would cause D6 + 6 - 10 points of structural damage. No secondary effects, such as the 'Eadbutt's stunning, apply against the Tank.