@Invector , Huh? For clarity, are you using a translation service for this forum? I just want to rule out translation errors in this discussion.
The game calculates an attack as a single action, with each source of damage getting it's own line in the combat log. Monster death occurs on whichever damage source takes it to zero health. So long as there's a fire damage in the attack, a troll won't regenerate, even if the fire damage wasn't the part that took it down to zero.
Example:
Let's say you're a Warrior that has a base damage of 2 (from traits, levels etc), that's using an Iron Longsword (Dmg 4 to 8).
The damage output per attack is 6 to 10. (Low 2+4, High 2+8).
Pretty straightforward right?
Now, you equip a Flaming Hatchet, which adds +2 Fire Damage as an off-hand weapon.
The damage output per attack is now 6 to 10, +2 Fire Damage. (As it would appear in your character sheet).
Low Damage: 2+4, + 2 Fire, for a total of 8 total damage
High Damage: 2+8, + 2 Fire, for a total of 12 total damage.
When you attack a troll, your going to get something like this:
You hit Troll, 6 -5 = 1
You Hit Troll, 2-0 = 2 (fire)
Line 1 is the physical damage. Your attack, randomly generated in the damage range. Minus the random armor value of the troll.
Line 2 is the elemental damage. In the game elements have colored font in the combat log. Red font for fire.
It does NOT matter if the 1 physical damage, or the 2 fire damage takes the troll to zero HP. Both will result in the troll not regenerating as the game checks to see if there was fire damage in the attack.
For clarity, that means if the troll was down to 1 hp, than the physical part would kill it. If the troll was down to 2, or 3 hp, than the fire damage would kill it.
If the troll has more than 3 hp, than it lives for the next round.
It also doesn't matter if there's another elemental damage at play, so long as there's one source of fire damage.
Let's look at a real example:
Attacking with a Nanocarbon Sword with Might Activated. The sword does 8 shock damage, Might does 4 fire damage.
In the following combat log entry, the troll gets the first hit in the round , but I get two hits as my weapon is slightly faster and with a slightly faster weapon you will sometimes get in an extra attack or two per round. Note, the game doesn't use formal rounds, but for organization of a combat log, the term is useful.
Ripped right from a combat log.
Troll hits you, 19-16=3
You hit Troll, 38-5=33
You hit Troll, 8-4=4 (shock)
You hit Troll, 4+1=5 (fire)
You hit Troll, 39-8=31
You hit Troll 8-4=4 (shock). Troll was killed.
Character Name: +99 XP. Companion: +61 XP.
Note: The shock damage in the second attack was what killed the troll. Because the attack had a fire aspect, the troll will stay dead.
Again, the fire damage COULD land the killing blow but it's just a matter of how it works out. Since fire damage tends to be the smallest amount of damage, sheer randomness indicates it will do the actual killing damage the least often (unless your a mage using a fire staff) but it doesn't matter.