A single word synonym for "A person not to mess with"

single-word-requests

I'm looking for a single word synonym for "A person not to mess with" without explicitly evoking a specific reason, as in "fearsome" for example.

Example in a sentence:

Kidd was an unfuckwithable pirate for my lack of a better placeholder.

I'd go with Formidable. According to Cambridge Online:

Formidable: adjective: causing you to have fear or respect for something or someone because that thing or person is large, powerful, or difficult:

Comparison with other answers so far:

Redoubtable: Means pretty much the same thing, but is more archaic.
No-nonsense: Means the person doesn't mess around. This is not the same as not to be messed with
Badass: Is informal, and directly suggests the person is violent. Formidable doesn't directly suggest violence. If you are a formidable chess player, then your opponents should have fear or respect of your skills; but you are not a badass because there is nothing violent about this. Another difference is "That guy is a badass!" usually means something like "That guy is cool!" and is usually positive, while "That guy is formidable" is not obviously positive or negative.
Awesome: The word has an archaic meaning of 'inspiring awe' which is similar to respect. However the modern meaning of the word is just slang for 'good'. In fact due to language degradation it's a pretty weak word for good.
Terrific: The archaic meaning is 'inspiring terror' which is like respect but different. The modern meaning is again just 'good' but even then it can sound a bit pip pip cheerio old chum British colonialism.

The formidable trees in Pokemon are formidable because they cannot be cut down. They are stronger than the other cuttable trees. If the stronger trees appeared as obstacles there would be no way through and the came would be impossible to finish.

I sense someone labeled a badass should be avoided!