How much damage can the Guardian of Faith spell deal?

Question

The guardian of faith spell description states that it disappears after it has dealt a total of 60 points of damage.

There are two things about that which I am questioning:

1. What about creatures which are vulnerable, resistant or immune to radiant damage? Do you factor in the amount of damage it actually does, or the attempted damage (10 or 20 depending on save)?
2. What happens if it has done 50 damage after several creatures have gotten close to it (e.g. one saved, two didn't), and then a creature enters its range and fails the save? Do they take 10 damage or 20 damage?

Answer 1

The rules differentiate between dealing and taking damage.

The Damage and Resistance section of the Basic Rules (emphasis mine) gives an example of such wording:

Resistance and then vulnerability are applied after all other modifiers to damage. For example, a creature has resistance to bludgeoning damage and is hit by an attack that deals 25 bludgeoning damage. The creature is also within a magical aura that reduces all damage by 5. The 25 damage is first reduced by 5 and then halved, so the creature takes 10 damage.

Example:

Emily casts fireball and rolls a total of 20 damage.
- A red dragonborn fails its DEX save. The fire damage is halved by the dragonborn's fire resistance, so the dragonborn takes 10 damage after reduction.
- A goblin succeed its DEX save. The fire damage is not reduced, so the goblin takes 10 damage without reduction.

The fireball deals 20 damage to the dragonborn, but only 10 damage to the goblin. The dragonborn and goblin take only 10 damage each.

The spell description says:

The guardian vanishes when it has dealt a total of 60 damage.

Failing the save means the guardian deals 20 damage, while succeeding means the guardian only deals 10 damage.

You factor in the outgoing damage towards 60, regardless of the damage the creature actually receives. The last creature failing the save on your example also takes 20 damage; after that, the guardian vanishes.

1. First enemy fails. The guardian deals 20 damage. Has the guardian dealt 60 damage? No. Then do nothing.
2. Second enemy fails. The guardian deals 20 damage. Has the guardian dealt 60 damage? No. Then do nothing.
3. Third enemy saves. The guardian deals 10 damage. Has the guardian dealt 60 damage? No. Then do nothing.
4. Fourth enemy fails. The guardian deals 20 damage. Has the guardian dealt 60 damage? Yes. Then guardian vanishes.

Total damage accumulated: 70 damage.

Answer 2

Guardian of Faith factors the actual damage done.

This part of your question was addressed in the Sage Advice Compendium, although in different terms:

[Q:] When a creature successfully saves against guardian of faith and takes 10 radiant damage, how much damage does that count against the total amount of damage the spell can deal? Is it 20 because that’s how much it dealt or 10 because that’s how much the target took?
[A:] It dealt 10 damage to the creature, so 10 is subtracted from the total.

The question implies that damage dealt is attempted and damage taken is actual. However, the answer ignores that implied distinction, because damage taken and dealt are one and the same: both are what you are referring to as the "actual damage done"; there is no such thing as "damage attempted".

Extrapolating for there, guardian of faith accounts for the save, for multipliers (such as resistance and vulnerability), for modifiers, etc. because what matters is the actual damage done to the creature.

Guardian of faith can deal more than 60 damage.

Nowhere in it's description does guardian of faith state that is can only deal 60 damage, instead it states:

The creature takes 20 radiant damage on a failed save, or half as much damage on a successful one. The guardian vanishes when it has dealt a total of 60 damage.

For example, say the guardian had dealt a total of 50 damage so far, then it dealt another 20 damage, and now it has dealt a total of 60 damage so it vanishes. In particular, although the guardian dealt 70 damage, that also means that "it has dealt a total of 60 damage" at some point in time (but that point in time is indistinct because the 20 damage is dealt all at once) and there is no indication that you need to retroactively reduce that last instace of 20 damage.