Just finished this one after numerous tries.
Once you have the mana, put a gork cage one space after the entrance of the mana corridor and a ballista at the space at the top part of that entrance. Then after the next mana surge I placed floor spikes right after the blades at the top (or bottom)corridor and a blade one space apart from the original blade at the bottom (or top if you placed spikes at the bottom earlier). The idea is to always have a ballista and 1 or 2 gorks to protect your mana while you alternate the floor spikes and blades at the top and bottom corridors. 3 stars always. Good luck.
