Mad Scientist Defence

The whole world is scared after the recent invasion of the alien invaders. They started with a major, strategically important, buildings, and after their destruction took up the rest. The aliens are very violent and do not take prisoners. But all is not lost, because a mad alchemist trying to build a bomb that exploded, only to destroy the aliens, and people and animals will not touch. Your task is to defend the lab scientist, until the bomb is ready.