Clicker Heroes

This should come naturally through level progression however you can get it idle.

Make sure you have a high DPS (damage per second), go to level 5 and just leave it idle until you get the achievement!

Bosses are found every 5th level/area. You have 30 seconds to finish off a boss before the battle restarts with full health. As you defeat bosses, you will progress through the following in-game achievements, symbolized by a red skull with horns and no jaw about midway down the achievement list:
* Boss Slaughter (Kill bosses 10 times)
* Boss Massacre (Kill bosses 100 times)
* Boss Exterminator (Kill bosses 1,000 times)
The achievement will unlock once Boss Exterminator is unlocked.

Tracking of progress in-game can be done two ways:
1) Selecting Boss Exterminator (it'll show progress as # of boss kills and % to completion)
2) Going to the in-game stats page, which has a "Boss Kills" counter (seventh one down)

If you pay attention, you may notice that the counter goes up whenever you beat any enemy at a boss level, even ones you have previously beaten. Thus, the speediest way is to advance a bit to higher areas, upgrading your heroes to do higher DPS. Once upgraded, drop back down to Drylands Lvl 5 or Drylands Lvl 10, where you should be able to kill a boss in under two seconds. Possibly even quicker. Given that it takes half a second for the boss to respawn, feel free to go to a higher level boss if you're killing them quickly enough. The bosses give more money due to their higher difficulty, which can help you upgrade more and be able to get farther within the game.

I made it to level 50 then went back 25. I simply left it there for a moment as it automatically killed the boss. Since my DPS is high it killed them instantly. The kills counted as boss and monster kills which will help with the "LEGION KILLER" achievement