Cover before my might

oh boi, what a *not* lethal weapon!

LOL

Then use a metal one

Gun: *melts*

*cough cough* it only affects biological things *cough cough*

*cough cough* plastic is made from natural gas and plants *cough cough*

OH-

A water gun full of venom would be very dangerous, but in order to cause real destruction you'll need a lot of it. And even if you have enough, you won't be able to destroy a building (unless it's a building made of biological material.)