I think the music would have to do a lot of the heavy lifting in this scene. Definitely need foley of the falling, but I think it could work to have the shot of the woman after she has been blastered if we can see the wound, just a circle of black with singed clothes around it, with maybe a wisp of smoke. It’s the classic delayed reaction. Cut back to Finn (without bloodmarks on his helmet) and the foley of her falling to the ground.

To sell the idea that it is the same trooper, maybe just have the shots of the flametrooper and Tekka walking through the flames then immediately cut back to Finn hovering over the woman as she dies so that it is clear that this is one continuous series of events. He also has a somewhat distinctive blood splatter on the jaw of his helmet which might be enough to distinguish him before the larger blood marks.