Well after completing the game 100% and completing the Valley of the Yetis, I must say the most challenging part of the entire game was the 3rd and 4th Shangri La missions. I almost nutted up trying to beat those lol. Those night missions in VOTY were difficult, well the last 3 were. Even if you had the entire defense built up. Just the sheer volume of enemies they sent at you made it difficult. What was the most challenging for you guys?