Interesting game, indeed!
Longfangs are mechdar's worst nightmare, but I think in your game you should've played on the strength of a 60" range gun sooner than you did. Deploying your Prisms away from your opponent's longfangs and placing your Serpents in a way that they give your Prisms cover from lascannon fire would have served you better, I think. Then going after the Longfangs with Prism AP3 blasts(even 1 or 2 dead per turn are a big help, you basically allowed them to shoot at you full strength whole game) and after the Razorbacks with Scatterlasers.
Longfangs are simply way too dangerous to ignore for any army, no matter the mission/deployment, but I guess you know that after this game!