Static Field works on anything that's not immune to lightning. However, in Nightmare it won't reduce enemies' health below 33%, and on Hell, not below 50%. So yeah, in Hell, the difference between a boss without Static and with Static is roughly twice as long. =p Lightning Resistance does affect Static, but still, unless you're chucking 30k Fireballs or something, Static will probably be your highest DPS.
However, the Crushing Blow mod found on some weapons has a similar effect that I don't think is restricted. (Crushing Blow removes 25% of current health on a nonboss enemy, 10% on a boss; half that if using a ranged weapon). Thus the highest DPS characters for killing Uber bosses quickly are ones with a very high Crushing Blow chance (possibly 100%) and very fast attack speeds (such as an Assassin spamming Dragon Talon or a Paladin with Zeal).