If successful, these moves cause the enemy to faint, regardless of
their HP, Defense, or Special statistics. Ordinarily, one-hit KO attacks
have a 29.7% chance of success, but Azure Heights researchers have
determined that a Pokémon's vulnerability to these moves depends on its
Speed, relative to the attacker. Specifically, it is known that:
1. Pokémon A will be only be able to affect Pokémon B with a one-hit KO
attack if A's Speed is equal to or greater than B's. The relative Speed
required for success with one-hit KOs is a matter of actual Speed, not
"base" Speed. Thus, a 30th level
Dragonair may be unable to affect a
Kadabra with Horn Drill. However, as
the Dragonair gains levels (and Speed), it will eventually be able to
affect that particular Kadabra.
2. Attacks that alter Speed can alter vulnerability to one-hit KOs. In
the previous example, if Dragonair knew Agility, it could negate Kadabra's immunity by
boosting its Speed sufficiently. If Dragonair knew Thunder Wave, it could also negate
Kadabra's immunity by paralyzing him (which produces a drastic reduction
in Speed). Conversely, a Pokémon vulnerable to one-hit KOs may be able to
achieve invulnerability by speeding itself up, or by slowing its opponent
down (again, success at these endeavors ultimately depends on the relative
Speeds of the Pokémon).
3. Speed alteration cannot be used to increase the chance of success
with one-hit KOs beyond 29.7%. Just because you have a
Rapidash with Agility and Horn Drill, don't expect to
somehow achieve an 80% success rate.
If a one-hit KO successfully hits an opponent's Substitute, the sub will
Guillotine and Horn Drill are functionally identical. Because they are
Normal attacks, neither can affect Ghost Pokémon.
Fissure can KO Ghosts, but it can't affect Flying Pokémon (due to its
Ground type). Otherwise, it works just like Guillotine and Horn Drill.
So when choosing between Horn Drill and Fissure for your
Mew, you'll have to decide which you
fear more: the speedy Gengar or the
(mostly) not-as-quick Flying Pokémon in the game.