The easiest way to make your character an expert is to make all your other characters fear him, respect him, or be defeated by him, in his field.
You characterize your Chief of the Bomb Squad as having won national honors for teaching Bomb disarmament, multiple medals for courage in the line of duty, a front line dude known to have risked his life to save children --- He encounters a bomb made by your MC Fred and says into his radio, "Jeezus Bill, Fred made this bomb, there is no way to disarm it. Evacuate the building!" Some kickback on the radio from Bill, and your Chief tells him "We've lost ten of our best trying ten ways to do it and I'm out of ideas. Evacuate the goddam building, and do it now!"
You don't have to say a thing about how Fred made this bomb: The reader knows Fred is better than his best opponent (the Chief you built up). Also, the Chief's explanation requires no special knowledge, he has to explain this to a layman (Bill).
If you need to reinforce this later, have some other Bomb Squad cowboy try to disarm one of Fred's bombs, in consultation with your Chief. The Chief says, "That isn't going to work, Bobby tried it and the second the capacitance changes it blows! Don't be an idiot!"
Cowboy says, "I know that chief, but I see the detector Bobby missed, and I can disarm it like ... this ... There!" KABOOM! End of Cowboy.
The reader can believe your MC is the best just because all the characters believe your MC is the best, and your MC keeps outwitting them. It is kind of grudging endorsement of your MC.
I should note you need to write other geniuses this way, too. For example if you are not a bona fide world class chess champion, you cannot come up with an actual chess strategy to defeat the world champion. You would need scenes where the best chess players in the world admire him and are defeated by him, without getting into the details of how exactly that was done. If the world champion is portrayed as uncertain and cautious facing him, the world champion has effectively endorsed the character.