One common default with many levels is that they are very easy until the last wave that destroys all.
If you like a challenge, try Sean's level 4 "Poison Eject" on difficulty Insane. This was possible to beat, on a previous version of Microbes, and I guess it still is. Work up to it from "Hard". I found this level very, very challenging, and so it's one of my favourite levels. Ideally every level should be just barely possible to beat on Insane, and this one fits the bill.
You have to cause the Microbes to go through as much poison as possible, keep your units alive, keep building extra units so you can survive the final waves, etc. It's not easy at the start, and not easy at the end. Much strategy and experimenting was needed to beat it.
You might need to edit your microbes save file to allow access to that level, if it's not enabled for you yet. If you find that microbes crashes out when starting, that's another time to edit your microbes save file! Keep a backup of this file so you can see what might have gone wrong with it.
Also, if you try some of Sean's later levels, I shuffled the epidemics randomly, so these will be hard right away at the beginning! You won't have to wait until the end for some major trouble! These levels won't be beatable on Insane difficulty, yet; and I'm not sure if that shuffling was a good idea.... we'll see.
_wb_ I reckon it would be good if microbes will set the current level back to 1/1 if it notices it's crashing at startup. Sometimes a broken user level can make the whole game unplayable, it tries to load the broken level at startup and crashes right away.