This level might seem extremely frustrating as you think you would need to kill every enemy during the messenger escort part, but actually you can just follow him without stress and revisit the earlier parts after reaching elevator cutscene.
First, kill everyone on the farm, then follow the messenger at your own pace and after successfully reaching the elevator, don't go in it yet. You can now backtrack the whole trail of the escort part or even go back to the farm if that's what you want, and kill everyone.
The most important part is that during the escort you don't destroy any of the barriers, because even if you destroyed them, after the cutscene, the barriers get resetted but the magic ball do not. Which means that if you destroyed them before the cutscene, you can't go back because you can't destroy the barriers now.