Nephthys
The Gr8est!!!!!!!!
Right, except you wouldn't use an AOE attack against a single opponent, you'd concentrate the energy into a focused attack. For arguments sake, Yoda wouldn't necessarily fare better than those 20 Jedi if Vitiate compressed the power that he would have otherwise spread out over a wide area with less power per target, into a single blast of lightning. Nihilus didn't blast a planet when he attacked Traya or the Exile, he only attacked them and not anyone else with a single, focused attack.
Collective potency still matters for an AOE attack. You need to put out enough power to defeat a certain amount of people at the same time. Collectively. To defeat 20 Jedi at once, you need to put out enough power to overwhelm the force defenses of 20 Jedi at once. It'd use the same amount of power as an attack that overwhelmed the defenses of an opponent with power equal to all 20 Jedi together. QED mother****er.
Good thing Nihilus doesn't actually use drain then. Remember that? That didn't stop being a thing that was true, despite our pet name for the technique. And we've seen Nihilus and Traya use it easily in combat situations. Derp.