The phrase "a weapon with which" is correct and usable in written English.
It can be used when describing a tool or means that can be employed to achieve a specific purpose, often in a metaphorical or literal sense.
Example: "He wielded a weapon with which he could defend his beliefs against any opposition."
Alternatives: "a tool that can" or "an instrument for".
In today's politics everything is a weapon with which to club the opposition.
"It should not be used as a weapon with which the younger generation can be beaten".
And she used her popularity with the electorate as a weapon with which to conquer the party.
And yet it won, giving tough-talking boss David Green a weapon with which to fight a legion of critics.
Mainly Mr Putin's belief that energy is a weapon with which to restore the lost greatness of the Soviet Union.
Cleopatra served most effectively as a weapon with which Octavian could club Marc Antony, in a particularly virulent civil war.
My dictatorial instincts have been roused by this week's gadget, a multi-use kitchen tool called the Stingray, which sounds like a Gerry Anderson-style weapon with which to battle the Aquaphibians, but looks like a spatula bought at B&Q.
Hence, we have one football player, Plaxico Burress, about to receive a two-year prison sentence for carrying a concealed weapon with which he accidentally shot himself in the leg and another, Donte' Stallworth, sentenced to 30 days in jail, two years of house arrest (which usually allows a person to go to work) and probation for killing someone while driving drunk.
It is a political weapon with which to divide and discredit an opposition movement that is threatening to undermine his party's near-monopoly on power in parliamentary elections later this year.
And for now, Wrigley Field gives them a potent weapon with which to fend off the White Sox's incursion.
Since it depends heavily on government loans to launch new designs, the politicians have a powerful weapon with which to defend jobs in their country.
