I just won the game and it lets me replay the end but I didn't get the achievement. Also, I didn't save for a really long time, so I can't just beat it again. This is shitty.
6 was crazy hard and I didn't come close to getting it until the one time when it was easy and over incredibly quick. Seems like a lot of luck is involved.
Impossible is actually impossible, although... You can force a stalemate by never evolving and spamming cavemen. You will continually rack up money and still be able to keep off your enemies, but you will be torn through quickly if you ever evolve. Eventually, you have so many cavemen that the game speed crawls to a halt. A moral victory.
Pretty sure the attack hit percentages are not accurate. I tried the 20 percent chance attack on the guard 25 times in a row without hitting. The chances of that are under 1 percent.
Turn down some stuff in the options menu, see if that helps :)