There are some "hacky" things you can do to raise the difficulty level such as
- giving the AI bonus resources
- giving the AI imbalanced units.
- giving the AI units better stats.
- giving the AI units that build faster.
these will not change the way the AI behaves, but will artificially make the game somewhat harder.
However, if you wish to actually have a level playing field in terms of the base game mechanics and just have the AI behave smarter - that is not a simple task. In essence it requires reprogramming the whole AI from scratch.
If you want to do that (and not have to personally program the whole thing) I think this system is the best thing available, but takes some effort to implement:
This tool originates from: http://www.wc3campaigns.net/forumdisplay.php?f=601 - Wayback Archive of Wc3c Source code now available at: SMUnlimited/AMAI Latest Release Notes AMAI is a mod/tool for Warcraft III: Reign of Chaos and The Frozen Throne made to improve the computer AI of the game in...
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