Bill Zebub
“It’s probably Matt Mercer’s fault.”
Ideally I think you need a monster with a lot of hit points, but a terrible chance to hit (I know they're out there, I just can't think of one off the top of my head). Put Starsky and Hutch in solo combat with the monster, calculating how many misses become hits, and at what point the +1 damage per hit matters (assuming the monster can't put them down before they win).
Well, yeah, the +1 becomes more meaningful against higher ACs.
I don't need a monster for solo combat: that's the whole point of fighting them against each other. Each is the other's monster. I just make sure they have identical AC and HP.
With 128hp and AC 20, Hutch wins 83% of the time. The longer the fight goes on the more time the swinginess has time to level out. In other words, at infinite HP Hutch will always win.