Increase the cost to 50(subject of balance) and let it have aoe effect limited to 6 supply, having the priority over units requiring the most supply.

In this case it could either frenzy 1 ultralisk, 3 hydralisks/roaches or 12 zerglings/banelings(practically as many as fits in the aoe radius) etc.

Infestor
Frenzy spell removed.
Infested Terran spell added.
derp?