There are actually 3 small and 2 medium sized tutorial levels, rest of the 15 levels are same larger size. The difficulty goes up based on how many targets need to be taken out, how fast repair bot repairs and how long the required number of targets need to stay been taken out. The difficulty should go up with each level, but there are intentionally easier levels at times and one before the last is more difficult than the last one.
There is no single solution to each level, but easiest tactic is to be stealthy at first and reckless later.