Idle Champions of the Forgotten Realms

This is a straight forward achievement that will come fairly quickly. Playing actively to upgrade my heroes a few times and then leaving the game running I was able to get this in around 5 hours after starting the game. If you just want to focus on grinding out these kills (which isn't necessary if you are going to go for other achievements) you can turn off auto progression and stay at an early level where you one shot all enemies.