The process was taking the 3 small shots that make up the entire shot, and fixing them up.
First, I needed a new background plate, so I stole one from later on and erased Malekith and corrected for the shot position.
Then, I just took the unfinished frame into Photoshop, painted out the dots, coloured his face and costume to match the rest of the scene, and add the new background.
There are two other shots that flash quickly as the lightning strikes Malekith. The first is just a flash of light on the actor and costume, to simulate Thor's lightning, as it would be added later by the VFX guys, and also myself.
The last shot is one of a slightly awkward facial expression as he is finally hit by the lightning, with the on-set lightning being turned up. I had to paint out the original lightning so I could add my own. This only lasts a single frame in the film so it's barely seen.
Now I just took the shots into After Effects and added a bit of lightning. Took some work to have the 4 frames to look like the final film, as the lightning core wasn't quite white and the glow wasn't blue nor white. I had to use a few layers to get it all looking right, but it looks alright to me.
The shot isn't perfect but for something that lasts 7 frames, its not too shabby. It's basically just animation at this point, not something too complex.