I’m not quite sure that I understand how the damage is calculated. I read the Game Guide and Combat Calculations Wiki entry.
Each skill has some base damage (I think), although most skill tooltips omit this information. Mana Strike states that it has 15 base lightning damage, whereas Shatter Strike does not mention it. According to the Game Guide, “Melee skills typically have 2 base damage total” - so I guess Shatter Strike does add 2 melee cold damage.
Now if my melee weapon does have 10 physical damage (implicit) and additional 10 melee fire damage (prefix), then one attack of Shatter Strike makes 22 damage, but only 2 of those damage points are cold - which means that the increased cold damage and cold penetration that I get from the Shatter Strike skill tree don’t do all that much. So in order to benefit from the increased cold damage, I would have to find a weapon with added cold damage. Am I right? Or I have to have lots of Dexterity and invest points into some passives, as that adds more cold melee damage. But still, having a weapon with cold damage would be beneficial I guess?
Mana Strike with Mana Storm is kind of a different beast, though. The lightning damage is (probably) spell damage and thus does not benefit from added melee damage. In this case, having adaptive spell damage (implicit on staffs and scepters) would probably help? Does it even make sense to invest into Mana Storm without such a weapon (as a Spellblade)?