Chanakya trying to make top or bomb?

i read it in my g.k book for competition of intelligence bureau,its too sad i didn't get the job because vacancies were very only 8 people in general category in india.sad me.nandas were the first people who used elephants in wars and thats why they succeeded in making a huge empire.chanakya knew it and he wanted to destroy nandas which could not be possible due to effective weapons and strategy.there is a name of that thing he made but i forgot it totally which hugely helped them in destroying nandas.