India side had long bow that create to penetrate wootz steel armor.That mean it can penetrate any personal armor in the ancient world. (Alexander was shot by bow at his lung bypass his armor at siege of Multanese.
http://en.wikipedia.org/wiki/Mallian_Campaign )
Weapon and armor of them made from wootz steel which was strongest metal in ancient world.
Strong elephant and cavalry army.
Large number of troop.Large population pool and strong economic strong industry that make them can reproduce troop very fast.
Alexander genocide some of India city that increase political support to India government and also make India force had to fight to the death.
Even from Greek source battle in India was more blood bath than in Persia .Alexander injure at Hydaspes river and his horse also die from this battle.He also injure again from siege of Multanese.
These states locate at hill terrain that mean their population density was low.Also total troop from these states was not much.
Chandragupta Maurya was beat Seleucus .That mean he can beat Greek army with Alexander best men.His Empire even was smaller than Alexander one but they had more population more advance technology and last longer.
With short supply and fertile land .I think Chandragupta could filed 100,000 men in battle. He can combine 30,000 bowmen 30,000 swordmen 9,000 cavalries and 1,000 elephants.This should be his best 100,000 men army so expect that most of them wore wootz steel armor.
Alexander side likely had 41,000 like his army at Hydaspes. Combine with 34,000 infantries 7,000 cavalries
Chandragupta start with use longbow to strike Greek infantry .And use swordmen to protect bowmen and use cavalry and elephant to protect their wing. Until Greek phalanx come close then move back and shoot again .They could did that because Phalanx could move very slow because their very heavy weapon.They could destroy Greek army this way.
Even Alexander can win this army by front engagement (which not likely) .They will face querrilla war that Chandragupta were expert at.At the end he would win by attrition war anyway .More population larger economic shorter supply line.