From what I've tested, damage bonuses are additive if they have the same origin(Mastery, Skills, Upgrades, Empower, Items) or multiplicative if they have different origins.

Dart damage test, you can check for yourself:
Base = 2
Mastery 101 = 12
Tier 1 upgrade = .5
Tier 3 = .5
Empower lvl 3 = (1+.1)^lvl
Arcshard T14 = .36

2*(1+12)*(1+.5+.5)*(1.1^3)*(1+.36)= 94.128
Ingame, it gives 94.1

So I assume ( for now) that it's the same on all other stats and traps

Also did some testing with some skills,
killchain works, tested with 101 Dart(2-2-1+3)[76,133 dmg], CuttingEdge(5points= 10%) and max killchain(40%) on a noble, removed 107 hp, compared to the calculated 106,59.

Redid the same test with a freezer with high slow and duration, max frostbite, expected either 213.17(multiplicative) or 182.72(additive), result was 107 hp removed, so it probably doesn't work or I'm doing something wrong. Need more testing.


Since items are way too random (Tier only increases the range, I once got a T20 Stygian Amulet that gave me +3 guardians, but my current is T16 and gives me +5 guardians.)
I'll put items on the sheet later.


Demons: Demon lords health is 5 times the strenght of normal demons and its damage scales from the normal demons mastery HP scaling.
Normal demons damage DO NOT BENEFIT from Cutting Edge, and probably Lich also doesn't.
Hex Demons hp are inconsistent, need more testing.

Finished a very rudimentar prototype calculator on my sheet, will update the online version later, but there are my findings.
Pure damage tier list with "optimal"(no items, no combos, no skills, only plain trap+ 101mastery+ upgrades) upgrades for max damage output against some 1E+13 HP enemies, assuming floor filled with NOHEALING slime, not taking into account pitfalls since it's an instakill anyway.
1v1 tierlist
Freezer(2-2-2)(assuming one of the enemies die and trigger the explosion) does 2E+12
Blackhole(1-2) (assuming it catches the enemy) does 1E+12
Teleporter(1-2) does 5E+11
Soul(2-3-2) does 1E+11

Horde tier list, assuming X enemies in range and no phasing(this can go to theorical infinite damage on area damage traps), DPSwise we have:
Freezer(2-2-2) vs 30 enemies(need only ~5 kills to cause a chain reaction with 100%hp damage, the problem is reaching there ) (same condition as before) = 3.6E+14 (yes, greater than the enemy max hp by 360 times, the problem is getting that chain started and all those enemies frozen, the bottleneck would be the rearm time + duration(rearm must be lower than half the duration to have a 100% frozen enemy upkeep) and a proper flow of enemies to keep chaining, if there's no flow, the chain will stop before the next wave comes and you will need to get more kills)
Blackhole(1-2) vs 45 enemies in range (3x3 with 5 each square) = 9E+12 (maybe it affects only one enemy, need checking, also it's 10% of current hp, so you would need 263 activations to kill )
Soul vs 45 enemies in range (3x3x5) = 4.5E+12(altough you would need 100 seconds to kill them, assuming no healing)

The best area damage is the shockwave, against a 1E+13 HP horde it does the greatest damage of 912K (or 9.1E+5 or 0.000009% of 1E+13), pretty bleak huh?

TODO list:
Upgrade the sheet so that it can use items and account for elements
Catalogue every item effect and put input option for their effect power
Test and update math on fire damage, dps and other variables(quite a bother since most tooltips give almost no data)
test more things
Redo math on barricade hp scaling with empower, it says 10% but I doubt going from 1.1x to 100kx in 3 empower levels doesn't go close to the 1.331x other traps get.
Lich damage dmg upgrade is wrong, it says 50% but goes only by 11%, need more testing
Need to check if grinder 1% per kill limit of 1000% is really of base or is a total multiplier(it seems like a total multiplier, but didn't manage to get on the 1K kills (only ~890) since enemies outscale the grinder cap regen way too fast. If the cap grew faster, it could be viable on 0-(low healing)Wrath runs(healing just puts a wrench on the gears lol)

Expectations of efficiency mathwise:
Burning gaze's kill on 10% hp will be of great help
Death's ash and fire in general ought to be good on high buffing enemies maps

Link to WIP Google Sheet[docs.google.com]