When fighting battles above your level, you'd need to fight defensively, and you'd only need to fight the part of the enemies that target you, the exact amount depending on how the AI is configured. That would achieve okay performance. Once you advance in tech, you can take more offensive approach, and face all off the enemies on the map, giving you a bonus in the final score if successful. As new types of enemies come, what is above your level and what is not changes. In the end, we would have both current offensive terror missions and new more defensive ones. The game would need to be balanced to work with that, but balancing takes work even without it.
If everything is configured correctly, playing defensively would be easier, but would only work in short term.