这句对考验加了总结：last（最后的，考完后面不考了），sharpest（最严厉的，较前两次更为严苛），ruthlessly（无情的，与sharpest呼应，表示必须的），long（漫长的，要15年）。
显然最后的考验筛选出两类人，一类人倒下了，另一类则经受住了考验。

And this last and sharpest test shall go on ruthlessly for fifteen long years.
而这最终且最严厉的考验，将冷酷无情地持续漫长的15年。

所在段落

The Story of Philosophy《哲学的故事》第1章Plato第7节VI. The Psychological Solution第17段第5句：

Alas! Not yet. Their education is still unfinished. For after all it has been, in the main, a theoretical education something else is needed. Let these Ph.D.'s pass down now from the heights of philosophy into the "cave" of the world of men and things; generalizations and abstractions are worthless except they be tested by this concrete world; let our students enter that world with no favor shown them; they shall compete with men of business, with hard-headed grasping individualists, with men of brawn and men of cunning; in this mart of strife they shall learn from the book of life itself; they shall hurt their fingers and scratch their philosophic skins on the crude realities of the world; they shall earn their bread and butter by the sweat of their high brows. And this last and sharpest test shall go on ruthlessly for fifteen long years. Some of our perfect products will break under the pressure, and be submerged by this last great wave of elimination. Those that survive, scarred and fifty, sobered and self-reliant, shorn of scholastic vanity by the merciless friction of life, and armed now with all the wisdom that tradition and experience, culture and conflict, can coöperate to give — these men at last shall automatically become the rulers of the state.
浙江大学译本：而这最终最残酷的测试将无情地进行十五年。我们这些完美的作品中有些人将经受不住重压而崩溃、淹没在这最后一轮的淘汰大浪中。

上句见：Day427