Background: I’m DM’ing a quintet of players whom I let accumulate a bit too much AC-boosting stuff and so are very hard to hit with weapon attacks (at 12th level they’re all near AC 25, and one has a Cloak of Displacement). For narrative reasons in the campaign, I need to send an elite solo NPC at them who can land several arrows in a surprise ambush attack despite their being invisible as they enter a large hall.
Constraints: I want to do this “fair and square” with a RAW-built NPC whose attack rolls can be out in the open on the table. A Ranger’s Conjure Volley spell isn’t good enough because a couple of the PC's are very good at Dex saves, and I want a very strong chance of them all getting hit.
Proposal: If am correct, a 20th level Ranger (Hunter) with the Archery fighting style is the ticket, if she has Oil of Sharpness applied to five +3 arrows and a +3 bow, and Wis 20 and Dex 20 (realistic with the 5 ASI’s a 20th level Ranger has). If I add this up correctly it is:
+5 (Dex)
+6 (Proficiency)
+2 (Archery Fighting Style)
+3 (Oil of Sharpness)
+3 (Magic Ammo)
+3 (Magic Longbow)
+5 (Foe Slayer feature, one shot per turn only, but can be chosen after the roll)
= +27 with Foe Slayer; else +22.
Using her Volley feature for her action, she can attack all five of them in her initial ambush (surprise round), with one shot of her (post-roll) choice getting a +27, and the rest +22.
This will use up five magical (and oiled up) arrows, and the players’ invisibility is ignored for purposes of these attacks due to her Feral Senses feature. At the end of all this, it’s fairly likely that she hits on all five shots, or darned near so.
Am I getting all of these calculations (and rules interpretations) correct according to RAW?
Best Answer
Your "to hit" bonuses are correct, but you may be overestimating Feral Senses
Your "to hit" bonus calculations are completely correct. As you indicated, this tactic will use up six very rare magical items (five +3 arrows and one Oil of Sharpness potion) in order to hit each member of the party once. That seems a rather expensive way to make a first impression, but I'm sure they have their reasons.
That being said, I wanted to warn you that the Ranger's Feral Senses have a couple of caveats that might complicate your strategy. You mentioned that "the players’ invisibility is ignored for purposes of these attacks due to her Feral Senses feature." If you meant that the attacks wouldn't have to be made with disadvantage, you were completely correct. But Feral Senses doesn't let you actually see invisible creatures: it just makes you so capable that you can still accurately attack creatures that are invisible. I mention the distinction because the rules on unseen targets state that:
Feral Senses will always remove the disadvantage on an attack roll against an invisible creature, which will help quite a bit. But it only lets you know an invisible creature's location under some specific conditions. According to the text on Feral Senses (PHB, p. 92, bold added)
You indicated that this tactic would be used as the PCs entered " a large hall," and that some of the PCs "are very good at Dex saves." This led me to believe that there are two possible confounds to your strategy.
1.) The Ranger might not know where Hidden (read: stealthy) characters are
If your characters are "very good at Dex saves," I assume that at least some of them may be stealthy. If the entire party is invisible, I assume that at least some of them are trying to remain silent as they move along. And since invisible creatures can always attempt to Hide (PHB, p. 177), if they roll better on their Stealth than your Ranger does on Perception, they will be hidden. And since Feral Senses doesn't let you know the location of hidden creatures, that means your Ranger will be guessing their location, or may simply not know they are in the room. This could lead to your Ranger missing these characters, in spite of his/her incredible bonus to attack.
2.) The Ranger may not know where characters are if they are far away in this "large hall"
It is accepted that unless characters are actively Hiding, it is fairly obvious where they are while they are in combat. However, this principle becomes questionable at longer ranges. If your Ranger is 100+ feet away from the PCs, it's a bit surprising that they can tell where the characters are from sound alone, especially if the characters are attempting to be quiet. Feral Senses explicitly lets the Ranger know the PCs' locations if they are within 30 feet. But at that range, it seems to me like there's a danger that the PCs will chase the Ranger down.
Naturally, as the DM you can declare that the Ranger is aware of the PCs location at any range you want: perhaps the Ranger is wearing a Robe of Eyes (DMG, p. 194), or has preternaturally acute hearing. And as the DM you are completely in charge of when stealth can reasonably be applied, and under what circumstances creatures are capable of hiding. But as you mentioned that you wanted "to do this 'fair and square' with a RAW-built NPC", I thought you should know about some of the RAW limits on the Ranger's Feral Senses.