I am level 58 right now, and I'm trying to collect stars in all of the chapters. But the sudden death is very hard and I only get low XP.
For example, for the chapter Desert Freighter, the most I've ever gotten is 34 XP. No matter the strategy I use, I only get 31-34 XP. But I never get a star, even at 34 XP.
What is the best way to gain XP? Should I avoid upgrading my towers, use only one tower or create a tower near my base?