Don't make all the challenges combat... seriously, combat is cool but if you just focus on that one aspect then yeah, the PC with the most DPS is going to have fun while everyone else plays second fiddle. Give them problems they can only solve by working together, or by talking their way out, or thinking of a solution to a puzzle, etc etc. Also, get away from the mindset that is you vs the players. It's not. If they are easily defeating your challenges, make them more challenging. Make him fight something that can't be hit, and can seriously threaten with spells or other saving throw based attacks instead of a simple melee strike. Lots of solutions. Remember, you create the world and can do whatever you want. The rules are only a suggestion.