A rod/stick to beat somebody with
Definition 1 of 1
Pro Tip 1/3Negative Connotation
This phrase usually implies criticism and is often used to point out someone's faults or weaknesses. 
Her teammates used her lack of experience as a stick to beat her with.
Pro Tip 2/3Metaphorical Use
The 'rod' or 'stick' is not a physical object but a metaphor for rebuke or reprimand. 
The auditor's report provided the media with a stick to beat the government with.
Pro Tip 3/3Not Literal
Don't confuse this with actual violence. It is a figurative phrase and does not involve physical beating. 
His poor performance reviews gave his critics a rod to beat him with.