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Changes the target's ability to Simple. Fails if the target's ability is Multitype, Simple, or Truant. Pokemon protected by Magic Coat or the ability Magic Bounce are unaffected and instead redirect this move back to the user. The effect is lost if the target switches out.
Simple Beam should never be considered for competitive singles play. There are very few Pokemon who receive this move, and not all of them are viable in any tier, including NU. The worst part of this move is that you are giving the opponent what is actually a usable ability. If a Pokemon uses Simple Beam on a Pokemon that can set up, the target can now set up twice as fast, and the ability it loses will usually not be necessary after it gains the ability to be able to sweep the Simple Beam user's team easily. In singles, Simple Beam is always outclassed by Worry Seed and Gastro Acid.
It's a different story completely in doubles and triples though. While finding the time to set up in the fast paced metagames of doubles and triples is usually impossible, should your team be based around setting up a sweeper, having a Pokemon with Simple Beam on your team is an awesome, and unexpected, tool. The best part is that it can act double as an ability remover like Gastro Acid and Worry Seed.
Worry Seed is an equivalent that gives the target the ability Insomnia.