This game has 10 levels so in order to get the hard badge you need to gain ~40k by level. It's not possible on the earliest levels but you should have at least 280k after the 7th stage if you want to have it.
How to win last levels without much effort: max your fence, build one laser and two missile turrets - max them out. Spend the rest on gunmen/repairmen with 2/1 ratio. About level 33 you should be able to click play and leave the computer to play without worry.