It would be very tough for Alexander. Chandragupta is younger than Alexander by a fair margin, and the age difference can play its part in ways that cannot be predicted on morale, battlefield performance and other things. Furthermore, India's population even at that time was simply too huge to take on, especially an unified India. Unless Alexander managed to mobilize the manpower of his entire empire against India (which would at best give him parity in numbers), he stands no chance. In a war of attrition, India would simply outlast his empire.