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:
+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?