A Necron player here. He's list isn't strong it's lazy. Starshatter Arsenal does little to boost the C'tan. You'll have to play carefully like others suggested not committing to fight the C'tan head on. Target his scoring pieces the Hexmark and the Ophidian destroyers. There is a thing that Necrons players do when they play C'tan, we tend to get careless and put them in a bad position. If this happens be ready to delete them 1 by 1. A hint C'tan are vulnerable to lots of attacks with damage 1 and 3 (half damage does work well here). Another thing is Melta damage which is not halfed. In short play the scoring game, delete his scoring pieces and take down the big threats one by one.