First, take a good long look at what the capabilities are of your four protagonist characters.
Then, ask yourself "What would be the hardest thing for them to beat?"
So, for instance if one of the hero mecha is a high speed unit, but does little damage. It shoudl go up against a slow unit that has area-of-effect weapons which render the others mobility moot. If a hero mecha has a nigh unstoppable uber beam cannon, the enemy mecha should have the ability to get out of the weapons way, or prevent the opponent from ever being able to target it, and so on.
Double points if you design it such that while hero mecha a could not beat villian mecha a, if hero mecha A fought villian mecha b they could win.
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Epsilon
Then, ask yourself "What would be the hardest thing for them to beat?"
So, for instance if one of the hero mecha is a high speed unit, but does little damage. It shoudl go up against a slow unit that has area-of-effect weapons which render the others mobility moot. If a hero mecha has a nigh unstoppable uber beam cannon, the enemy mecha should have the ability to get out of the weapons way, or prevent the opponent from ever being able to target it, and so on.
Double points if you design it such that while hero mecha a could not beat villian mecha a, if hero mecha A fought villian mecha b they could win.
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Epsilon