You asked for nit picks. Here is my (TWO CENTS)
When someone is standing right behind the person in front. the flame only hurts the person in front but the particles (APPEAR) to be going through the person in the back. So I would either stop the particles if it hits something or hurt the person behind.
It appears it should be getting the person because their are standing in the flames and the flames around around them but they are not hurt until (HP) hits 0 on the person in front.
At the start of the sequence, the person catches on fire before the particles (APPEAR) to hit the person. The detection seems to be on the first particle but (VISUALLY) it is a bit soon.