3 sentences

1. 这种做法万万不可。

   zhè zhǒng zuò​fǎ wàn​wàn bù kě.

   This practice is absolutely unacceptable.

   Source: Dong Chinese Dictionary

2. 这件事情关系重大，万万不可外泄。

   zhè jiàn shì​qing guān​xì zhòng​dà, wàn​wàn bù kě wài xiè.

   This is a matter of great importance and must never be leaked.

   Source: Dong Chinese Dictionary

3. 面对如此强大的对手，我们的防范万万不可太疏。

   miàn​duì rú​cǐ qiáng​dà de duì​shǒu, wǒ​men de fáng​fàn wàn​wàn bù kě tài shū.

   Facing such a powerful opponent, our vigilance must never be too lax.

   Source: Dong Chinese Dictionary