To beat the 100 (without 10,000 gold start) you really need a three path maze to juggle enemies through. Secret is the upgrade order of your anti-air, which should get the most priority since you can bounce the ground guys back and forth all day. Final hint: you will need to employ snap towers starting from level 77 or 85 on to beat all the flyers and about four snap towers on level 98 to kill off enough of the flyers to survive.