My protagonist is (at least once he has gained self-acceptance) superhumanly clever (he's the incarnation of a trickster god). How do I write that and be believable?
Slowly. Give yourself time to come up with ideas that your protagonist thinks of immediately. Also backwards. Think of a clever solution and then come up with an appropriate problem that requires it.
For a trickster, cleverness is often about being able to read other people and spot their weaknesses. You have a big advantage in writing this, since you created those other people and already know them inside and out. Show how he interprets their behavior and evaluates their natures, playing to the things they want most in order to ensnare them
Lots of rewrites. Lots of rewrites. They give you the hindsight to see what a truly clever person can find on the spur of the moment.
If you want to write something that appears to be more intelligent than you are, then you need to utilize the tools around you....embrace Google Seriously, the internet allows us to be smarter than what we actually are. In RL, I commonly use 'redneck' slang and am a simple minded person but when I am doing my college papers, focusing on moving resources in a mass fatality incident (emergency management scholar), I write such good papers, that I get a full A every time - because I have mastered my ability to research.
Be careful you don't make everyone stupid by comparison. I seen this happen in someone's WIP- they wanted to make the mc look very smart and tried to do it by contrast and it kind of backfired. Not everyone is collectively stupid. I agree with Alred James Taylor - working backwards seems to work for a lot of mystery writers and they always look clever.
Do your research. Also, read other work showcasing characters with the level of depth and intelligence that you want to instill in your character, for inspiration. Get inside their head.
Hi, I'm going to agree with the others about working backwards. It's all about working the logic of what people can know. As an old maths teacher of mine once said being a genius just means that you're able to reach the wrong answer faster than other people. And a name of caution - Lex Luthor. This character in my opinion is a perfect example of what you're afraid of. Think of Lex and what he always seems to know. Then ask yourself this one question - how can he possibly know that? And the answer too often is that he couldn't. The greatest mind in the universe can't know things that he doesn't have the basic facts for. So Lex could not possibly know that for example kryptonite would be Sup's weakness any more than he could possibly know that a particular Zulu warrior who he'd never met was allergic to grapefruit etc. Cheers, Greg.