protection vs. -1/-1

I was told by someone who usually knows what theyre talking about that a spell like mutilate or marsh CASUALTIES goes right through protection from black. Is this true?

Yes because it doesnt target the creature directly. The spell targets the opponent. (you ment marsh casulties right)

"Protection" protects the permanent from being targeted by a spell/ability of said colour. "Protection" protects the permanent from any damage from a source of said colour.

Neither Mutilate nor Marsh Casualties are targeted and -1/-1 is not considered damage, even if it destroys the creature.

Just remember DEBT

D For creatures with protection, all Damage that would be dealt to it from sources of that quality is prevented. For example, if a creature has protection from black, black creatures can't deal any damage to it, nor can any other black cards deal damage to it.

E The card cannot be Enchanted or Equipped by cards of that quality. Again, a creature protected from black cannot be enchanted by black enchantments or black equipment. If a creature gains protection from the quality after it has already been enchanted/equipped by that source, the attached cards will detach and go where the rules would normally force them to go in that circumstance. For this reason, white enchantments that grant protection from any color tend to state that their effects cannot remove themselves should you name white (or the enchantment's color is changed to whatever you named).

B Creatures of the stated quality cannot Block the protected creature. So, a creature with protection from artifacts could not be blocked by artifact creatures. This does not prevent the protected creature from blocking those creatures, but they cannot block it. For example, White Knight cannot be blocked by black creatures, but it's still free to block other black creatures when they attack you.

T The creature can't be Targeted by sources of the named quality. For example, Putrefy could not target something protected by black, green, instants, or whatever other qualities it happens to meet. This applies even if the creature didn't originally have protection, but gained it before the spell/effect resolved. If you tried to play Putrefy on an otherwise normal creature, and Bathe in Light was played in response to name green, the Putrefy would be countered on resolution because its target is no longer legal. Keep in mind that this only applies for targeted spells/effects, which means it has to say "target" on it. If it doesn't target anything but just affects it, protection does nothing. Wrath of God is a common example of something that could get through, say, protection from white

What protection doesn't do:

The above effects are all that protection does, but there are some common mistakes regarding what protection does. The biggest one that comes up is regarding non-targeted effects such as Wrath of God. Black Knight, even though it is protected from white, can still be killed by Wrath of God because it is not dealing damage, enchanting/equipping it, blocking it, or targeting it. White Knight can still die from Mutilate because it's neither targeting nor damaging. Just because it's a global effect doesn't mean it's guaranteed to work, though. Pestilence will not harm a black-protected creature because it is attempting to deal damage.
Protected creatures are also not immune to cards that specify that creature without targeting. Clone can copy a creature protected from blue because it only requires you to choose a creature, not target one. If a black-protected creature blocks Gorgon Recluse, it will still die by the effect because it's not targeting anything.