It been way too long since I played this game so I cannot remember mission specifics... However, the first few missions are usually the easiest. You should not have too much problem taking the upper hand.
If you really cannot corner the AI then remember that there is always a fixed amount of spice on the map. The first thing you should do is build two refineries and sit tight until your develop your base and have enough units to make a strong attack. If you don't win in the first wave keep attacking them in waves and they will eventually fold to your superior money gathering power.
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