Additional True Damage/Shroud

Assassin Sniper deals physical damage. Shroud passive deals portion as True Damage.
Green Desert Guardian deals magic damage, plus additional True Damage.
True Damage is made up of physical & magic combined. But when True Damage is result of passive or a skill, do the characters stats need to be increased? Example: Assassin Sniper ATK is high, is there benefit to increase Magic stat? Or does passive/skill automatically account to inflict True Damage?

Not too sure, but I'd like to add one additional question-

If you have a true shield talisman set, does it reduce the damage converted to true damage by the shroud effect?