1. This is your case, it's entirely up to you, this is just friendly advice.
这案子你接了 完全归你管 只是要给你提个善意的醒

up to
由...决定

eg:I give you blessings; Happiness is up to you.
我给予你祝福，快乐幸福是你自己的选择。

2. He was killed by a single blow to the back from a blunt instrument.
他是脑后遭钝器袭击 一击毙命的

blunt
钝的，不锋利的

eg:Too much alcohol made the young driver's sense blunt.
大量喝酒使得那个年轻 司机感觉迟钝。

3. He's trying to be clever. It's overconfidence.
自作聪明 自信心膨胀

overconfidence
自负，过分自信

eg:Overconfidence in oneself is the first step to fail.
过度自信是走向失败的第一步。

4. Low self-esteem, tiny IQ and a limited life expectancy.
自信不足 智商低下 胸无大志

expectancy
期望，期待

eg:For most Americans, life expectancy continues merrily to rise.
对于大多数美国人来讲，寿命预期持续喜增。