There's no great way to grind this, I recommend focusing on every other achievement first. Don't forget that the game is still in early access and updates will come out adding more features. I got to about 450k kills while doing every other achievement until I said I'll grind it.
During the last 50k kills picked the Spellblade character with the epic weapon, equipped the Singular focus, and fire or swing (or both) runes. Went to a map with no curses, stacked the main ability till I had all slots filled with it. After I filled my slots with only the Firestorm ability and made the AOE big enough to let me afk through the bosses and stages. Looped until I died and repeated this process. It took a bit but it was a very effective strategy.