Alright, lets get this outta of the way, you can take the long way of grinding the game for 100+ hours on idle or sit there with some sort of auto clicker but even with that, at a certain point, your idle progress is strong enough to carry you through everything.

I reached that point. I could kill any mob, easily clear 300+ etc, etc.
I still needed that dumb, time consuming achievement of 1,000,000 Kills.
I no longer wanted to grind or play the game anymore as the interest was gone. Better things to play.

SO, what to do?

%appdata%\Roaming\MMFApplications

In there, you should find a ini file called x1nntp
Open it up in Word or some other basic text program. (Notepad, etc)
Edit whatever you need to in order to get your achievement or achievements in case you're still missing others.

Under [Stats] you want "EnemiesKilled=999999" to be displayed. Once you open the game up after saving the file, you'll get the achievement to pop.

Now you can also use this for other achievements like Chests, Stage completion, Hero levels etc
For me, it wasn't needed.