Yes, Your Grace

I did it with 600+ troops but in another playthrough I did it with 500+ so if you have 600 and use the agents correctly you should win it. When the green smokes come ask the which to remove it with magic, when the monsters come use the hunter and again horn and archers and when the fire starts use the general. If you have more troops you can miss some of the agents help.

A visual representation with commentary of the achievement in the video.