There is an extra calculation going on that explains why Finger of Death is often so useless.
First, of course, there's the chance of the spell itself. You need master skill 20 to get 100% here.
Secondly, the enemy can resist it. The calculation for it to connect is 30/(30 + MonsterResistance + MonsterLevel). So as you can see, higher level monsters are more likely to resist it. This is also the case for spells like Mass Distortion, Stun, Slow, Paralyze, etc. etc.
So if an enemy is level 30, even without any resistances, it will only work half the time... and the higher level the enemy is, the less likely it becomes that it will work.
In fact, Mass Distortion is even worse: not only do you need an effective earth magic skill of 38 to deal damage equal to 100% of the monster's hit points, the monster can also resist the spell outright with that 30/(30 + MonsterResistance + MonsterLevel) calculation, but also! if all that goes through it can still resist the damage as if it was a regular damage spell. And all that is assuming the monster isn't just immune to magic anyway.