Hmm, strange, but I really haven't found where can I change difficulty level of the game. I think it has just one setting.
In this mission they give you around 5 minutes (or maybe less) before first attack or they will attack almost immediately after you start harvesting.
It is not hard when you know what to do and are careful about your ships and resource management.
I have never had time to build 10+ platforms, having just 2 didn't have much impact, but I do think I shouldn't underestimate their importance, as that pulse platform is almost always 1 hit kill for smaller ships, though it is slow.
And in my case laying mines wasn't the only solution, I managed to build more balanced fleet. Instead of 1 torpedo frigate I built 5-6.
