For starters, Illumise cannot learn Tail Glow so your strategy just went down the drain
Secondly, the assumption in which Substitute does not break is a kinda low chance one, due to the low bulk on both of them, especially when pitted against powerful attackers in doubles
Thirdly I'm pretty sure Simple Beam is blocked by Substitute. Just gave a quick trial run on PS, (I decided to use Volbeat), and it's Special Attack still went up by 3 stages.