DISCLAIMER: This is not a comprehensive guide. This was an early analysis just after Phase 2 was released. Many more things have been discovered since that are Mayhem scaled (Hollow Point, Fire in the Skag Den…), scaling keeps being changed (TTB), and there’s weird stuff going on with some bonus elements, and the list goes on. This is just an early snapshot.
Now that action skill and melee damage scale with Mayhem, I ran some tests and these are the early results. I’ve tested Phaseslam, Phasecast, Fist over Matter and Ties That Bind. TTB aside, all other action skills I’ve tested follow the same pattern. That same pattern apparently also applies to Rakk Attack (thanks @plenipotence), so I assume that’s the base pattern for AS scaling.
So here are the values:
As you can see, AS and melee damage bonuses increase proportionally with enemy HP bonuses, except for TTB. But all of them (including TTB) are calculated based on enemy HP bonuses. I figured out these formulas:
Action skill damage (Phaseslam / Phasecast / Fist over Matter / Rakk Attack):
DmgBonus = HPBonus × 0.3
Meaning that for every +100% enemy HP from a Mayhem level, your AS damage is increased by +30%. That’s +60% AS damage at Mayhem 1 and +3,000% at Mayhem 10.
DmgBonus = HPBonus × 0.15
Same principle, only that the increase is half that of action skills. This is +30% at M1 and +1500% at M10. This also applies to Fish Slap and Face-puncher.
Ties That Bind:
DmgBonus = 100% + HPBonus × 0.05
TTB apparently has its own weird unique rules, it ranges from +110% at M1 to +600% at M10.
edit: Moze’s Short Fuse apparently scales the same way. Thanks @Prismatic
edit 2: this also applies to Indiscriminate. Just like TTB, it used to receive bonuses from Do Harm, but now it receives a flat x7 bonus on M10 instead.
Here’s a chart showing the multipliers for each Mayhem level:
Now TTB looks pretty poor on this chart, but don’t let that fool you. Keep in mind that this chart shows Mayhem scaling multipliers, not actual damage dealt. TTB damage is based on actual damage dealt by anything you hit the enemy with, instead of the puny base damage of other action skills. In fact, on M10 TTB does far more damage now than before, even though they removed AS damage bonuses from TTB.
Do Harm still increases damage from Slam/Cast/Fist over Matter, but it no longer increases TTB damage. Don’t bother with Do Harm anymore in a TTB build.
Not convinced? Consider this: TTB now gets a flat +600% bonus at M10. Previously, if you had 99 Do Harm stacks with 5/5 Do Harm and 3/3 Awakening, you got a bonus of 99 × 4.5% × 1.3 = +579%.
On M10, TTB now deals more damage out of the box than it did with 8 points invested in Do Harm + Awakening before. Don’t even start complaining about a “nerf”.
All these bonuses are “multiplicative” as far as I can tell right now, meaning that e.g. if you have +275% melee damage or whatever, the +1500% on M10 will multiply the result from that.
BaseDmg × (1 + 275% + 1500%) = BaseDmg × 18.75
BaseDmg × (1 + 275%) × (1 + 1500%) = BaseDmg × 60
I may update this post if new things are found.