According to what we seen, the enemy is not just a step ahead, but a whole mile! Karna and the Demon lord taking the slow approach to make those guys into "heroes" and not interfering because the contract/word given to the now dead hero, would definitely bite them in their asses!
But why to worry when Karna will, again, perform some miracle to defeat the bad guys!
Just don't think about all the people who die in the meantime