A number of factors affect the damage of Skeletal Mages (Singularity)

Intelligence [1 = 1% multiplier]
Resource Cost Reduction [10% (Paragons), 4 essence saved per cast]
* Total Essence [50 (Paragons), 20 (Weapon), 20 (Offhand)]
Area Damage [20% (Shoulders)]
Raw Damage [Weapon, Off-hand, Rings, Amulet]
 Attack Speed Increase [7% (Gloves, Belt, Rings)]
 Increased Attack Speed [7% (Weapon)]
*** Critical Hit Chance [5% (Native), 5% (Paragons), 6% (Offhand, Helm, Bracers, Gloves, Rings), 10% (Amulet)]
Critical Hit Damage [50% (Native), 50% (Paragons), 130% (Emerald in Weapon), 50% (Belt, Gloves, Rings), 100% (Amulet)]
% Increased Skeletal Mage damage [15% (Helm, Boots, Offhand)]
% Physical [20% (Amulet, Bracers)]
% Increased damage to elites [8% (Weapon, Offhand)]
Bones of Rathma set [4000% (4 mages)]
* Reilena's Shadowhook [0.5%/max essence (330)]
Tasker and Theo [1.5]
Convention of Elements [200%]
Krysbin’s Sentence [100%, 300%]
Bane of the Trapped
Enforcer
Zei's Stone of Vengeance
Bane of the Powerful
* Skeletal Mage [200% per hit, 3% per remaining essence]
Simulacrum [double mages, doubled essence pool (dynamic)]
Oculus Ring [85%]
Corpse Lance (Brittle Touch) [100% CHC]
Strongarm Bracers [30% each, from 2 sources (additive)]
Gem of Efficacious Toxin [10% each, from 2 sources (additive)]
Rend (Mutilate) [10%]
Threatening Shout (Falter) [25%]
Frenzied [25% added attack speed (multiplicative with Speed Pylon?)]
Power [250% added damage]
Speed [30% added attack speed (multiplicative with Frenzied Shrine?)]

Remarks

* Double Dipping: Essence is considered twice in calculating the mages’ damage (directly on cast, based on essence consumed by skill; and maximum essence as a multiplier on Reilena’s Shadowhook). This makes essence a very valuable resource.

 The attack speed of the mages can only be increased by Tasker and Theo. All other attributes related to attack speed are factored into the damage calculation, but do not increase the actual attack speed of the mages. The difference between “% Increased Attack Speed” (IAS) and “Attack Speed increased by %” (AS) works the same way as other builds, with the former affecting the based attack speed and the latter as an additive modifier.

* The probability of each hit by the mages being a critical hit is modified by CHC (I know right… but please read on) but CHC is not integrated as part of the damage. This is in contrast with skills like Meteor (Star Pact), where critical hits are integrated as the hit damage [1 + (CHC x CHD)] before taking the probability of the meteor damage actually proccing a critical hit into account. In this sense, the Meteor dips twice from the critical hit pool. As a result, running high CHC as a DPS Rat is only beneficial if the zNec does not have sufficient time to apply a full Corpse Lance (Brittle Touch) debuff. In other words, a decent amount of CHC is good for clearing trash, or when the zNec is running a CDR-restrictive build that does not allow for much crit capping. As a corollary, there is no clear benefit of focusing on CHC over running more attack speed, raw damage or CDR if the runs focus on elite sniping, or is at a low enough difficulty where trash melts.