Ishq is love, and Dhishkyaun is a simulation of a gun's sound with mouth, like an onomatopoeia. So while Dhishkyaun is here used to signify the sound of a gun, ishkyaun is used as a portmanteau of Ishq and Dhishkyaun, making it signify 'fight with love'.
Hence, when the hero says yahaan ishqyaun wahaan dhishkyaun, he means that while he's fighting with love, or simply showing love, she's fighting with guns against him.
The idea also translates into a scene where Deepika shows a gun and Ranveer shoots with a pichkari, a water-gun.