A common lament I've seen with writing boils down to "I want to write a strategically smart character but I am not strategically smart." It's okay. You don't have to be George S. Patton III to write a clever-clogs. You just have to be aware of the moving pieces in your own book (and it's your own book, so I hope you're aware of them). The strategically smart character can read between the lines and see the enemy's flaws. You've already written them down. Find the flaws that have been put on display somewhere that character would be aware of them. Then build a plan around exploiting them. This does two things: One, it establishes your Sun Tzu as smart enough to pick things up that aren't explicitly told to him. B, it reinforces the existence of the flaws in your antagonist, making them consistent traits rather than one-off mistakes. Third, it gives you clear checkpoints for the plan itself where things can go wrong but the underlying understanding still leads to a measure of success for your smarty-pants. As an example: You have a character who knows the enemy commander in a siege. They're not exactly good friends, but Mr. Brain has seen enough of the enemy and his culture to have a good idea how he thinks. Maybe he's a student of history and has an idea of how the two peoples involved have related in the past. He now has a solid grounding for what to expect. The enemy has surprises and is able to take tactical victories. But because Mr. Brain knows, in general, how the enemy thinks, he is able to plan around the tactical loss to turn the final outcome into a victory of some sort, close or pyrrhic. He can plan twists and surprises that aren't pulled out of a butt, but rather develop naturally as a consequence of everything that's been happening. Say the enemy shares a pantheon with the defenders; Mr. Brain picks the temple of his enemy's favorite god as the fallback point. He's now exploiting his enemy's reverence to buy himself a little extra time if he needs to make a quick escape. Hopefully you've already established that all the temples are linked by tunnels or something that allow our hero to escape. The key to coming up with a clever strategy for a clever strategist ultimately boils down to knowing which pieces you've put on the chess board, and then exploiting them. If you hit a dead end and there are no pieces left, then you need to slip another piece onto the board somewhere earlier in the book. You don't have to be a strategic genius to write a strategic genius. You just have to know more about what's happening than your characters, and then let your strategic genius character figure out more than everybody else has.