In short, not at all.
It is true Electrify would make all incoming moves useless against any Ground type/Lightningrod/Volt Absorb user, but there are three main factors that severely limits it's usefulness.
The first of these is that the user of Electrify must move before its opponent. This is especially damning to Stunfisk which is a very slow Pokemon, meaning most of the time it would simply be sitting around doing nothing whilst a myriad of Pokemon attack before it can do anything.
Secondly, as you have to use Electrify you cannot use any other move that turn, meaning you have no way of fighting back. Sure Toxic Spikes or Sandstorm can be used to chip away damage, but that is both slow and unreliable at best. At the same time your opponent have no reason to be inactive, as they can set up or simply switch around, wasting your PP.
Thirdly, you might be led to believe Electrify would be good in Double battles, where both of the opponents Pokemon would be rendered useless whilst your second Pokemon can set up freely. That also falls short, as Electrify only effects one Pokemon at the time on the field, meaning it is more or less completely useless, even in double battles.
So in medium length, nothing would be OP with Electrify because it leaves you wide open, and that is even if you move first. You become a sitting duck, that might be annoying to hit, but nothing else.
