I am currently on overload 12.
Before the new patch I could barely complete overload 11 and only with help of good random bonusses I got through.
I almost got overload 12, but some of the enemies have way too much armor/health. Even with best weapons it seems impossible to kill even one big enemy. Part of it is also that the waves now are more mixed and the towers do not get any chance to recover after a wave of shooting enemies. Before patch, some waves only consisted of fast moving enemy, giving me a chance to rebuild. I liked that you could get a flawless if the enemies only hit the shield. That gave me more flawless bonusses and could make it possible to get a few more overloads.
Can not imagine how to get to overload 20 without 100% perfect bonusses with the current build.
But I like the game and will not stop until I get all levels :)