Question

What type has only one weakness?

Answer 1

Electric and Normal Types only have one weakness, those being Ground and Fighting respectively.

Comments

Electric being weak to Electric, seems legit.

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Yep. 100% legit.

Answer 2

I'm going to add that water/ ground when combined has only one weakness. Same with steel/ bug. Dark/ ghost has no weaknesses. Dark/ poison and Dragon/water have only one. Also ghost/ normal (no Pokemon has been made that is ghost/ normal yet, though). I'm just listing a few combos to go with redshift's answer

Comments

And dark/poison (2x weak to ground) and water/dragon (2x weak to dragon) Factoring in its non dreamworld abilities, bronzong has only one weakness. sap sipper seaking is a similar case. factoring in abilities and as of yet unmade type combos, there are more. Example: normal/electric with levitate, grass/steel with flash fire, etc.

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I forgot about dark/ poison, but I wasn't counting abilities

Answer 3

If I remember right, dragons are only weak against either ice or other dragons. Normal and ghost can't hit each other, poison can't hit steel, psychic can't hit dark, and electric can't hit ground. Hopefully that helps.