Writing dozens of characters into a story can add depth to the tale, but at cost of making it hard for the reader to follow along. A cast of thousands can also make the plot itself seem like an afterthought.
Most stories that are getting a little overcomplicated can benefit from cutting out some characters. However, this can result in characters who’ve inherited all the traits their ancestor-characters had.
When you have a character who’s good at everything, how do you make them believable and sympathetic? If you’re asking yourself this question, you may well have a characterization problem.
Ask yourself, could your character have reasonably picked up their portfolio of skills within a normal human lifetime? If not, you probably have superhumans on your hands, not human beings. Before you pull your cast apart into its component skills, there are a few other things you can try first:
Sherlock Holmes was believable because he was an unbearably obnoxious person who loved to show off his genius. James Bond is also a bit of a polymath, but he was impatient, careless, and a misogynistic womanizer; in both cases, these characters are pretty much infallible in their fields. (Almost.) We suspend disbelief, but Holmes and Bond are firmly two-dimensional. Does your character have vulnerabilities that would make them three-dimensional and believable?
A lack of social skills (or a lack of empathy) can easily fill in for a character who knows far too much. Learning the social graces takes time and energy, and polymaths would be too busy educating themselves to have picked up how to politely tell someone they’re not interested in going to their formal dinner party. The cost of acquiring a skill is the lack of a different skill.
Try rewriting the character to take out a few skills; or better yet, make the character a little shaky in some of them. Why should life be easy? If all problems are solvable, there’s less conflict and danger. In the right situation, the skilled pilot is much less interesting than Ted Stryker. (Who was more of a neurotic wreck than a polymath pilot, but you get my point.)
If you realize you’ve gone too far in reducing your cast, you might reverse things a bit and split just one super-character into two. That introduces other problems, and if the tale is character-based, you might kill any momentum you’ve got going. But this is a real option if you’re in the early stages of writing this story.
Finally, you can add some backstory to make these skills believable–but be careful with this one. You want to hint at why the protagonist knows how to fix a jet engine with a Leatherman while it’s running, you’re not here to tell an unrelated story about that time in a B-52 flying over hostile territory with an enemy agent trying to sabotage the mission. (See how distracting that is?) A fun backstory can easily take away from the “A” plot unless it’s very carefully crafted to move the main plot forward.
Ask yourself, what would have driven someone to learn all these things? Why would they give up so much to be good at so many things?
If you still can’t make up your mind about whether or not you have a problem character in your story, you can “decide” to temporarily not worry about it. Take advantage of the momentum you have going and finish the book, put it in a drawer, and look at it with fresh eyes in a week or two. Don’t let completion be the cost of perfection. (Number one piece of advice for writers: Keep writing!)