2 sentences

1. 这是一个千载难逢的良机。

   zhè shì yī gè qiān​zǎi​nán​féng de liáng​jī.

   This is a golden opportunity that comes once in a lifetime.

   Source: Dong Chinese Dictionary

2. 这是一个千载难逢的机会，你一定要好好把握。

   zhè shì yī gè qiān​zǎi​nán​féng de jī​huì, nǐ yī​dìng yào hǎo​hǎo bǎ​wò.

   This is a once-in-a-lifetime opportunity; you must seize it.

   Source: Dong Chinese Dictionary