Mad Courage is one the most difficult achievements to get in King’s Bounty: Dark Side. It’s not as obvious as it seems, so here’s my help, as some people can struggle with getting it.
How to Obtain Mad Courage Achievement
To get Mad Courage you have to defeat the Spirit of Light in the final battle without using your Spell Book.
In that case you rather not try it with Daert the Vampire, cause it might be a little bit painfull.
And these are not all the conditions, since it seems to be affected by character skill – tactics.
You can still have it, but using it in the battle will waste your chance for the achievement.
Personally I recommend you playing Bagyr the Orc to get more value out of units and because it’s most certain and easiest to get this achievement by him.
Also, from my experience, unit skills do not affect the Mad Courage, so feel free to use it. Good luck!