The first 6 are melee piercing damage targetting the char. The burning is untargetted fire damage. The burning may be magic damage instead of melee... I think that was done to stop Barbed Platemail from injuring its owner.
The first 6 damage are melee piercing, as written on the card. The following damage are melee fire, which means Polearm Slash can reduce the damage and Shimmering Aura cannot.
I hate it when that happens...more damage than the armor simply failing. At least it's penetrating, so it can't trigger again, right? ...
its in this context noteworthy that the fiery stab fire can be upped by the firemage traits (and instant burn)... In case you like battlefield training shenenegans.