The scene: A 10th-level College of Swords bard (includes Extra Attack). Crossbow Expert. Sharpshooter. 20 Dexterity. Hand crossbow +1.
Intelligence is his dump stat so he needs to ask a friend what to pick for his Magical Secret.
One is definitely Find Greater Steed, because he is going to ride on a griffon while raining down death with his hand crossbow. The other may or may not be Swift Quiver.
- Greater Invisibility will give me 2 attacks with my action, 1 attack as a bonus action, +10 to hit with advantage.
- Swift Quiver will give me 2 attacks with my action, 2 attacks with my bonus action, +10 to hit.
Ignoring opportunity costs, spell slots costs, defence and anything else other than crossbow damage per round, at what point is it better to use Swift Quiver vs. Greater Invisibility?
Best Answer
Greater Invisibility + Sharpshooter is usually best
I'm sure someone else can come with an analytical solution to this, personally I'm fond of the universiality of monte carlo methods. The general principle, create a simulation of what you want to compare, run the simulation many, many times and average the results.
In this case, all we need is a function which for a set of paramaters (number of attacks, advantage, AC, etc.) simulates that round (rolls to hit and for damage). Run that simulation 100000 times for each set of parameters. So, for each of the four cases (GI or SQ and Sharpshooter or not) we can run that for a set of ACs. My Python code here. Doing that we get damage per round as a function of AC for each case (GI is Greater Invisibility, SQ is Swiftquiver, and a suffix S means with Sharpshooter):
For AC 13 to 22 using greater invisibility with Sharpshooter gives the highest DPR. So unless you're fighting exceedingly low AC enemies or exceedingly high AC that combo is best. If the AC goes 23 or up just use greater invisibility until you hit AC 29 at which point you should start using Sharpshooter again (you're only hitting on crits, so you just want hits to hit as hard as possible). Using swiftquiver and Sharpshooter is only better for ACs 12 or lower, at which point you're basically hitting every non-nat1, so advantage isn't worth the one less attack.