I'm considering a Fighter 1/Monk 1/Rogue 18 for a character. Our DM has a few house rules that I want to take full advantage of by combining Great Weapon Fighting's rerolls with Sneak Attacks.
Specifically, the rulings involved are:
- Monk weapons have the finesse property when wielded by a monk.
- GWF's reroll applies to extra damage dice, not just weapon dice.
- Multi-classing is permitted, obviously.
- Flanking rules from the DMG are being used.
Now, a monk/rogue/fighter can combine these like so:
- Use a versatile weapon two-handed (e.g., quarterstaff), to activate GWF
- Flank an enemy to enable Sneak Attacks
- Attack using +Dex, gaining bonus damage dice from Sneak Attack, which can all benefit from GWF's benefit of rerolling 1s and 2s on damage dice
The question is, how does this stack up against other known good builds?
The two benchmarks I have in mind to measure this against are the Great Weapon Master + Polearm Master strategy, and the Sharpshooter + Crossbow Expert strategy. I know these are strong, especially if one has CA and (for the Polearm one) has good use of action economy to get more DPR.
And if I got a hold of a Sun Blade (DMG p. 205), with its inherent finesse property, I could do without the 1-level dip into Monk. Would that significantly change how the build compares to the benchmarks?
Best Answer
Let's work out some of the math here; the details of your build and your DM are going to influence how these numbers work out. Some assumptions I am making based on what you have stated.
Assumptions
The Math
My first thought is that the thief build is going to be really top heavy; you are trying to get as many dice rolls as possible because you want that natural 20 critical to double the number of dice you are rolling for damage. I did a little look at monsters vs stats and I am trying to figure out a good threshold for when a die roll will hit and I feel like 13 on the die should hit based on average AC of monsters at that level. So 13 for die roll, 2 for weapon bonus, 6 for proficiency and 5 for stat. Now one could argue these numbers one way or another but I am going to be using them as the base because I need something to compare apples to apples.
So 13 + 2 + 6 + 5 = 26 for target AC to hit or 35% chance to hit and a 5% chance to crit. I used http://andrewgelman.com/2014/07/12/dnd-5e-advantage-disadvantage-probability/ as a reference for determining advantage so that translates to a 63.9% chance to hit and a 9.8% to critical. That makes it so 54.1% of the time you are going to do normal damage and 9.8% time you are going to do double damage. I basically am saying 2/3 of the time you are going to hit with your attack, really anything that gives you 2 attacks is going to let you get a sneak attack in during that round so long as you are within 5 feet of your target. This is also a pretty high number, some could argue that 10 or above would be a more realistic number then you are talking almost an 80% chance to hit with advantage.
Since you decided to dip into fighter instead of go 20 rogue you don't get Stroke of Luck but that would only be a factor depending on how much resting vs combat you would do. To ease math on calculating average damage I am assuming your weapon damage for rogue is a D6. If you are getting a D8 or better this just slightly makes your numbers better it just helps me to do my math faster.
Critical Hit - 18d6 + 2d6 + 5 + 2 = 77 Avg
Normal Hit - 9d6 + 1d6 + 5 + 2 = 42 Avg
GWF Critical Hit - 18d6 + 2d6 + 5 + 2 = 85 Avg
GWF Normal Hit - 9d6 + 1d6 + 5 + 2 = 46 Avg
The math was kind of surprising because of the limited reroll only on 1 or 2 and you could get a 1 after you rerolled a 2 but on average on 10d6 rolling a 1 or 2 and getting 4 extra damage on average.
Polearm Master
After looking at the polearm master the numbers are basically the same for attack and damage except that you get another attack that you will get your critical damage against. Even though the base dice is a d4 that extra attack is what gets you the key advantage — do you want that extra damage from the reroll or the chance to attack again and be able to sneak attack? Frankly I'd rather take the 63.9% to do my sneak attack dice again then the extra 4 damage on average.
Crossbow Expert
This basically follows the same rules as Polearm master: the feat is letting you get an extra attack with the a crossbow and I still think it is better then Polearm master because being able to attack at range is crucial; however, that depends on your flavor and what you like to do. However where this starts to break down is flanking and how your GM rules on flanking with a crossbow. If your GM rules you can't with a range weapon then you are stuck with polearm.
Bottom Line
Bottom line your GM house rule about flanking is getting you the most bang for your buck here because you want to maximize that rule. The winner between all 3 is really hard to decide, and also depends on how your GM house rules critical hits and determining which one was a sneak attack. If you get to pick the extra attack is going to be always worth it. If you can't pick which attack to apply sneak attack damage to then they are really almost a wash because your extra critical chance vs the on average higher damage from your rolling is going to end up being almost a wash in the long term at high level.