I concur, and I'm playing on impossible.

What file I need to change to edit its mana cost? (and where that file is located)

The folder is ....\sessions\ice_and_fire . Open the archive ses.kfs and and then open spells.txt . The name of the spell is devatron.
I guess I'll check it next time I play , in my last 2 games I never found it so I don't have much experience with it.

Thanks.

By the way, I cannot beat the Spider boss. How many attemps do you do before giving up? What tactic and strategy do you follow to defeat her? I don't care to be spoiled, please tell me how the heck you can crush the spider, I was enjoying the average difficulty till this point, I'm desperated.

I'm level 13 Viking hero with 29 Attack (19+10), 13 Defense, 14 Intellect. 38 mana. 17 Warrior maidens, 137 pirates, 22 Assasins, 57 Berserkers and 11 Royal Griffins, with 14 soothsayers and 68 bowmen in reserve.

Hi , glad to hear it's challenging :) . But that's why I said impossible is not possible without knowing how the game develops.
First of all, I don't attack the spider before getting to level 15 , even 16 if I manage to do it. And that also means longer fighting with the units you choose for the spider fight - which means they will increase their rank(very important). You can't kill it with units ranked 2-3 .
Second, there were always some options to have a level5 unit in your army. I think I managed to kill the spider without level5 once,can't remember the details. My last game was with mage hero and I had level5 , so it's different from yours.
Third, you have to plan everything since the beginning,thinking exactly for that encounter and choose every skill very carefully.
Hope this helps :)

Yes it does. My units are all above rank level 5, though. Didn't find level 5 creatures..

Is there any workaround to make this easier? I don't want to restart, honestly. I think I could reach level 14 with several loses, but probably not more. Getting up to level 15 would imply facing "Very strong" or deadlier encounters.

I think you can find level 5 units in Merlassar and Ice Gardens before fighting the spider boss.
Edit: You can't.

There is no workaround :) . I think there were some lvl5 by the witch in the pirate's cave. But anyway by that time you will not have enough fights left to up their rank. So I guess restarting and planning carefully is the way to go on impossible.

To Space Knight - This mod does not allow you easy access to Merlassar and Ice Gardens. You have to beat the spider and then some more to get there.

Ok. To be honest, I still haven't played the mod (planning to), so I just took a guess.

You can always use this cheat: Have a stack of Witch Hunters in your army. Cast the talent that gives damage each time enemy uses a skill, cast it on the big spider. It dies in 1 round... it's a vanilla bug. I don't know if jorko80 fixed this, though. I used it in vanilla.

Hahaha :) , awful BUG . Never heard of it. I'll fix it in the next version of my mod.
Thanks raknefne!