It is Ijma of Sahaba and the Ummah that Imam Ali is the most courageous and bravest amongst the followers of Mohammed Mustafa (Sallal Lahu Alaihi Wa Sallam). Just study the seerah and you will see Badr, Khandaq, Khayber, Hunain etc. For instance, in Badr, 72 kuffar were killed and 37 were killed by Imam Ali (Alaihi Asalam).
The origins of the Naara are from the battlefield and it is recited in praise of the one on whose hand Allah Taalah granted victory to the Muslims after others had failed. When Hazrat Ali (Alaihi Asalam) once defeated many foes by himself, and the Sahaba’s (Radi Allah Anum Ajma’een) said, Nara e Haideri – Ya Ali (Alaihi Asalam).
Sayidna Ali (Aliahi Asalaam) is known as Haider – Lion
Ya Ali (Aliahi Asalaam) is the reply for Naaray Haideri.
Naaray Haideri is the call for Sayidna Ali (Aliahi Asalaam) since he is Asadullah.
Sayidna Ali (Aliahi Asalaam) is also Mushkil Kusha, thus we say Ya Ali to call upon Sayidna Ali (Aliahi Asalaam) as a waseela (intermediately or intercessor).
Sayidna Ali (Aliahi Asalaam) is Imamul Auliya.