Here's the project file. You can mess around with it and see the results you get. Let me know what you did so I can make my render better.
To get rid of the noise in the volumetric you would need to increase the number of samples considerably and this is probably not practical as it already takes a considerable amount of time to render.
There are things you could do to improve the render times.
The volumetric domain is quite a bit larger than it needs to be. You could scale it down in the X dimension to that it is sufficiently around the lightening but not too far towards the camera or too far behind. This will reduce the volume that rays need to pass through it, considerably improving render times
The trees are very high detail with many polygons and transparency - you could simplify the trees to reduce the number of faces. Possibly even rendering a small set of trees and adding them to the scene as an image plane, layering multiple trees to get the complexity.
The lightening seems to be very high poly - with 6 levels of subdivision. You could re-model the lightening with better geometry to considerably reduce the polygon count.
However, you would still require a large number of samples to resolve the noise - you're already rendering at 800 samples and may need to increase that by a factor of 10 or more to start to resolve the noise to satisfactory levels.
An alternative is to remove the volumetrics entirely and, instead, use the compositor to add the effect of the scatter using Blur and Fog Glow effects.
To demonstrate, I rendered out your image as two separate layers - the lightening :
And the trees (but note that the trees have not rendered with the same detail as your sample image - I believe this is due to your images for the pine needle leaves not being packed into the Blend file). It should be sufficient for the demonstration though :
The 'brightness' of the lightening is actually due to two distinct effects - the 'scattering' in the atmosphere (which makes the sky appear brighter around the flash) and the 'glare' in the lens (whether camera lens or the eye). The main distinction being that the 'scattering' light will be behind the trees while the 'glare' is in the lens and so can appear in front of the trees.
This can be achieved with the following compositor nodes :
This is a fairly simple setup and some additional tweaks could produce better results (such as feathering the edges of the mask with a blur and shrink/grow for better transition between trees and sky).
Essentially, this setup uses the Depth pass of the trees to generate a mask that can be used to separate sky from trees. A Blur node spreads the light from the lightening to mimic the scattering (the Darken node cuts down the amount of light since the emission is too strong and would 'white out' the entire sky). The mask is then used to mix in the lightening behind the trees and glare added and the brightening of the sky (when unmasked) is added in.
You can adjust the Factor of the Darken node, the Threshold of the Glare and the dimensions of the Blur to tweak the effects.
This produces the following result :