I answered a similar question a while back, and my reply there will help you get a sense of how Qin became a great power in the long run.

In terms of equipment, tactics, and leadership, the Qin army was not very different from the armies of the other Warring States. All of the late Warring States armies were built on a core of levied troops armed primarily with spears, polearms and crossbows, supported by chariots of various types and cavalry. The states all used essentially the same types of weapons, armor, and siege equipment, and the same strategic manuals (including the Sunzi Bingfa and the Wuzi) were available to all of them. Qin Shihuang had the services of a very capable general, Wang Jian, but the rival states had skilled commanders of their own, like Li Mu of Zhao. In short, the Qin army did not have any apparent qualitative advantage over its rivals, whether in terms of military technology, strategic or tactical knowledge, or the ability of battlefield commanders.

Where the Qin army did differ from the other states was in how it rewarded its levied soldiers for battlefield performance. From the late 4th century BC onwards, Qin had practiced a Legalist form of government in which a system of harsh punishments and generous incentives were used to prod Qin subjects into behaving in ways that most benefited the state. In order to encourage the common soldier to be aggressive and persistent in war, the state offered grants of land, social rank and legal privileges as rewards for heads taken in battle, and to the poor farmers who filled out the ranks of the Qin army, the rewards that a distinguished military career would bring could be life-changing. This was one of the most promising paths for social mobility in Qin society. The end result was that, as noted by the author of the Wuzi military treatise, the common Qin footsoldier was uncommonly aggressive compared to levied troops from other states because of this promise of social and economic advancement.

What overall impact this had on Qin's battlefield performance is debatable. The author of Wuzi recognizes that Qin troops were aggressive and highly-motivated, but also notes that their hunger for individual achievement undermined discipline and unity, and could be used to lure them into traps or bait them away from their commanders. The author didn't seem to think much of soldiers from Qi or Chu, but also had praise for the soldiers of Yan, Zhao, Wei and Han, although in different regards. Ultimately this may not have been as large of a factor in Qin's eventual success as the geographical and institutional factors I described in my earlier post.

In any case, Qin Shihuang (born Ying Zheng or Zhao Zheng) personally had little to do with the Qin unification. Qin had already become a superpower under his grandfather, King Zhaoxiang, and the reforms that built Qin's political and military powerbase had been carried out a century before that, during the reign of Duke Xiao. All that was required of Qin Shihuang was to press Qin's advantage against each remaining state in succession.