[Kromm]

One has to treat Leech like Innate Attack here and simply have it inflict its injury on everyone affected. Healing should be proportional to total HP robbed. However, note that Leech has an innate limitation: "You can only affect one victim at a time." Even if you're some 100-armed super-grappler, you cannot grab 50 victims and use Leech on them all; and even if you have Extra Attack 19, you cannot zap 20 people with Leech (Malediction 1, +100%; Ranged, +40%) to drain them. This is intrinsic to the advantage itself, not to its means of delivery – Leech can only process one HP stream at a time. To get around that, you would need Cosmic, +50%.

Even leaving off Ranged, look at the sheer expense of Leech with Area Effect 1, +50% (you can't add Emanation, -20% to get points back, as Leech without Ranged has no range in the first place), Cosmic, Multiple victims, +50%, and Malediction 1, +100%. That's +200%, or 75 points + 12 points/level. I think that's rather hard to abuse.