Ultratron

Letum is the "final" boss which you fight at the end of level 40. Letum is a large sphere with guns on all sides that will follow you around the room with two consecutive dashes followed by a rest period. You can most easily beat him by moving around the edges of the area in a circle to take out the guns on all sides. You have to make sure to not get stuck in a corner by his dashes so having the Speed Boost ability with the Quantum Shift upgrade is recommended by this point.