Can you make a spell attack with the Hunter's Giant Killer?

Question

The full Hunter's Prey ability is written as such (Player's Handbook p.93, emphasis mine):

Hunter’s Prey
At 3rd level, you gain one of the following features of your choice.
Colossus Slayer
Your tenacity can wear down the most potent foes. When you hit a creature with a weapon attack, the creature takes an extra 1d8 damage if it’s below its hit point maximum. You can deal this extra damage only once per turn.
Giant Killer
When a Large or larger creature within 5 feet of you hits or misses you with an attack, you can use your reaction to attack that creature immediately after its attack, provided that you can see the creature.
Horde Breaker
Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.

I've highlighted the reason I'm quoting the whole feature: Horde Breaker and Colossus Slayer both specify the abilities require a weapon attack, but Giant Killer does not. Could it reasonably be argued that the ranger could instead cast a leveled spell or cantrip, as long as it is a spell attack, as a reaction?

Answer 1

You can make a spell attack, but you can't cast a spell

First: I will agree this is probably a small typo in the text, and that the intended reading is for Giant Killer to be for weapon attacks only. Disregarding that for now and looking at only the RAW text provided:

Casting a spell may result in an attack being made, but is not the same thing as making an attack- a feature that simply allows you to make an attack does not allow you to cast a spell to do so.

A very simple comparison: When you make an attack of opportunity:

you use your reaction to make one melee attack against the provoking creature.

This is functionally the same case- opportunity attacks do not specify 'melee weapon attack', merely 'melee attack'. Still, this does not permit you to cast, say, booming blade, even though that cantrip would directly result in you making a melee weapon attack (or shocking grasp, for a melee spell attack). We have had such clarified in the Sage Advice Compendium:

Can you use a melee spell attack to make an opportunity attack?

You can’t if the spell attack is created by casting a spell. When a creature triggers an opportunity attack from you, you can use your reaction to make a melee attack against it. The opportunity attack doesn’t suddenly give you the ability to cast a spell, such as shocking grasp.

Each spell has a casting time. A game feature, such as an opportunity attack, doesn’t let you bypass that casting time, unless the feature says otherwise.

How can you make a spell attack, then?

There are spell attacks that do not require you to actively cast a spell. A few examples:

• flame blade (if spell is already active)
• vampiric touch (if spell is already active)
• Sun Soul Monk, Radiant Bolt feature

The Sage Advice compendium also notes that some monsters have innate spell attacks which would qualify for "spell attack but not a spell". This is unlikely to be relevant for a giant killer ranger, except perhaps in the very niche scenario where you are shapechanged into another creature (which retains your own class features).

Answer 2

You can make a spell attack, but there are no spells that meet the conditions required.

The Giant Killer ability doesn't specify it needs to be a weapon attack, and while I would put money on that being the intention, for now let's assume it would be fine to make a spell attack in response. The problem comes in that there is no clarification or adjustments to the potential spell attacks, like is seen in the third bullet point of the War Caster Feat (Player's Handbook p.170, emphasis mine):

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

The spell attack we would be able to make would need to be a spell that:

1. is a spell attack, ranged or melee, and
2. has a casting time of one reaction.

We immediately run into the problem that there are no spells that meet both of these criteria. Even if there were a spell attack made as a reaction, all spells made as a reaction have triggers tied to them, like "...when you or a creature within 60 feet of you falls" or "...when you are targeted by a spell." This specificity leads me to believe even if a new spell was created that meets both of the above criteria, it would not work with the Giant Killer feature unless the very specific triggers for reactions were intentionally lined up with this feature.