Well... I am not really sure that Marcelon would win the battle against Qin... As mentioned before, many things plays a part in the battle. And obviously I am not saying that Qin would slaughter the Marcelon Army either... that would just be boastful.
As far as I can tell, and from reading from history books, the Qin Army is one of the strongest in ancient world. The Marcelon Army are also standing at equal footing as compare to the Qin.
What ever we can tell are all from recorded history, no one was to know for sure how good each army was. And it was not always that the army who broke through Persian, Indians, etc are the best in old world... they are the best, might due to the fact that the others might be too weak. And from what I have read, King Phillips and later his son Alexandra the Great, actually introduce an army system that based not on conscription but as professional troops, that is why they could win. It is like fighting peasants.
As for the Qin Army, well... many are conscript as Zaver had pointed out... but as I have also pointed out, Qin Army are at war for 200 years. And so every men in the country had actually seen plenty of battles. They too have regulars and elite forces, that are heavily armoured and they too have palanx. and as pointed out, crossbow are easy to use weapon, and even the untrained could use them to some efficiency. Lets say, one soldier could fire ten in a minute, what would one thousand of them do. And what would a couple of thousand do.
At that time when horses do not have stirrup, fighting on the horseback is quite difficult or outright impossible. However a crossbow could actually be utilise in a single hand (although only one bolt would be fire at the most), but in a cavalry charge, each soldier on the horse back could fire once, bringing large number of arrows at enemy...
So as I could imagine, to defeat the Qin is not as straight forward. But to say that the Qin could easily win over the Marcelon is also not possible... (I am not very well verse with the Marcelon army, but from my limited knowledge, I do know that they are unstoppable in ancient world.)