I'd turn Phantasmal Killer into an Intelligence Saving Throw like Phantasmal Force is. For consistency, and Int is less resisted I think.
But the real problem is that a Concentration spell has to compete with Larian's equipment (+1d4 Radiant/Fire/Poison, dip, coat, Ring that adds +2 Acid damage etc.) and absurd Haste rule changes for damage output. It can't.