How to beat last stage of tower defence lost earth?

I am having a bit of trouble with raven stage aka E8, i can manage the wave that is coming in but the question is i can't find a weapon that would reach the third and forth roll of purple things to destroy it for me to reach the boss i try all the weapon and max it out and still could reach it. Is there a special place and weapon to set up to reach the third roll of purple thing?