I cannot get past wave 40. Theres just too much enemy spam all my ballistas get overwhelmed. Ive tried a lot of priority settings and clusters vs spaced out placements, but it just never works. Lost a LOT of games there, especially ones that loop around the center tower so i can evenly level up all the ballistas but the result is always the same. Only strat i have left is to sell the ballistas around houses at that point and place them to focus on the tanks near the portals.

that one seems the hardest to get, GJ