1. I outlined the area where the BFG shot should have burned through the Serpents armor. I imagined it would be a huge laser/plasma type weapon and the pointy-eared pilot had barely managed to dodge the shot to avoid complete destruction.
2. I took out my trusty Dremel-copy with a circular blade and cut away a piece somewhat along the lines I had drawn.
3. I smoothed out and angled the edges with a carving tool so that they would blend in with the stuff I filled the hole with.
4. I filled the hole with some sort of foam people also use for flower arrangement. It's light so the Serpent should still stay relatively straight on the flying stand. I first though of using milliput but that would have really messed up the balance of the mini and it would fly tilted.
5. I applied some Vallejo White Pumice on the foam. I thought it would create a nice charred texture when painted black, and it did!
6. I glued some plastic sprue shavings and some of the random molten plastic bits that were left from cutting the hole on some spots, mostly around the edges of the hole. It will create small bubbles and molten looking marks when burnt. I also glued some bigger bits in the middle to create interesting looking texture.
7. I glued a few small pieces of styrofoam on the damaged region, some of it on the pumice, some on the plastic. Styrofoam shrinks a lot when heated so these can be about the size of your fingertip or even bigger depending on what kind of effect you are looking for. It's also a bit of a surprise how it behaves when heated as it might just vanish almost completely, or it might melt or just create something resembling the pumice texture...
8. I let the glue dry for a while.
9. KILL IT WITH FIRE!!!!
(9. Go outside or somewhere else where you won't burn down your house, cause fire alarms, or choke to death with all the toxic fumes you'll be releasing soon. Be careful not to hurt yourself and so on... Light a candle, hold your model so that the damaged region is facing downwards and bring it close to the candle. Stuff will start to burn and melt, be careful not to completely burn your model or create unwanted damage to it.)
10. Scrub away any ashes and other burning residue with an old toothbrush. Try to be gentle, otherwise your battle damage might get damaged with some fiddly bits falling off.
11. Paint it nice and pretty somehow.
I'm not an expert on the matter by no means. This was all done on bit of a whim without proper testing

It's just how I thought it might work if I get lucky. The other option would have been a ruined model...