In the span of a month, Dwarven officers had been assisting in training the Demon Clan's army, mainly in aspects such as forming up, musket drills, and formation changes. Through their tireless efforts, they finally made the Demon Clan's army look somewhat presentable.
It should be noted, though, that the Dwarven officers had not directly advanced them from the era of cold weapons to the firearm era. Instead, the Demon Clan soldiers were already from the firearm era, so the Dwarven officers were able to train them up in just a month.
If it were an army from a purely cold weapons era like that of the Dongsheng Empire, conservative estimates would suggest that it would take at least about a year to train them into line infantry, not including the time needed for equipping and outfitting with new gear.
Because the Demon Clan's army already had a foundation, with the support of Dwarven officers who also came from the line infantry era, the training naturally bore fruit effortlessly.