Thanks!
So, the damage was probably calculated roughly like this:
12% of total health with the medal bonus (taking the maximum because of a critical hit), 18% with the 50% extra damage from the critical, then 2.7 times that amount because of -170% fire resistance gets us to 48.6% of total health. Yummy.
And I did not have any viking talents to increase the critical hit damage even further. Maybe there is some way to increase the critical hit chance of burning?