I've done some calculations that indicate that death effects could be treated like maximized disintegrates that only affect creatures subject to death magic. That's 12 hp/caster level. But that's mostly to explore the notion that a really tough creature or character would only be hurt, not killed, by a failed save.
Since disintegrate is a 6th level spell, you could try a formula like damage = spell level x caster level x 2. A phantasmal killer cast by a 7th level wizard would do 4 x 7 x 2 = 56 hit points if it takes effect. Kinda nerfs it, but I suppose that's the intent.
An implosion cast by a 20th level wizard would do 9 x 20 x 2 = 360 hp damage. An old red dragon has 378 hp, so this is pretty significant.
To nerf it further, you could say that only 10% of damage applies once the creature is reduced to zero hit points. So a 20 hp character who fails vs a phantasmal killer is reduced to -3 hp. A 300 hp dragon who suffers the full effect of an implosion is at -6, and so on.