@flannan
sure guns are a great force multiplier but they require a industry build around it... if they invented guns it would take atleast a couple years for research and development. after that they have to design ammunition then they have to form assembly lines for the parts or have them craft made which would take even longer not to mention getting gunpowder and the metals . not just that ,guns would only be effective against the grunt/lower level demons and would not work well against larger enemies and it would take a long ass time to develop a gun capable of killing one while staying practical .
on the other hand magic is quite well developed in this world and rhere is probably some magic than can work better than guns the issue is only specially trained individuals with years of experience can use them ,where as anyone can learn to use a gun in seconds
early muzzle loading cannons on the other hand would be much easier just cast steel and you can use almost anything as ammunition but cannons are heavy and hard to aim with mediocre accuracy they are also prime targets for aerial enemies
most of the tactics i listed can be easilly utilized by a nation of their level(~1500AD) and would only be hindered by man power rather than by technology and knowledge