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?

The rule differentiates between dealing and taking damage.

The Damage and Resistance section on the Basic Rule description (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 roll 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 takes 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 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 fail. Deals 20 damage. Has the guardian deal 60 damage? No. Then do nothing.


2. Second enemy fail. Deals 20 damage. Has the guardian deal 60 damage? No. Then do nothing.


3. Third enemy saves. Deals 10 damage. Has the guardian deal 60 damage? No. Then do nothing.


4. Fourth enemy fails. Deals 20 damage. Has the guardian deal 60 damage? Yes. Then guardian vanishes.

Total damage accumulated: 70 damage.

Guardian of Faith accounts for all modifiers and multipliers.

A similar question was asked about vampiric touch and the argument is essentially the same.

There is no distinction between damage "taken" and "dealt", except for the subject of the sentence: targets take damage from sources, and sources deal damage to targets. Damage taken and damage dealt are one and the same, just described from different perspectives.

For example:

The first creature is vulnerable to radiant damage and fails it's save so it takes 40 damage and the Guardian deals 40 damage.

The second creature is resistant to radiant damage and passes it's save so it takes 5 damage and the Guardian deals 5 damage.

The third creature is immune to radiant damage and so it takes 0 damage and the guardian deals 0 damage.

Overall the Guardian has dealt a total of 45 damage.

The Guardian can deal more than 60 damage.

The spell does not say that the Guardian can only deal 60 damage, just that it vanishes once it has dealt that much.

To illustrate this, lets continue the previous example with the guardiant that has dealth 45 damage.

The fourth creature fails its save and has no relevant resistances, vulnerabilities, or immunities so it takes 20 damage and the guardian deals 20 damage.

Overall the Guardian has dealt a total of 65 damage so now it vanishes.