Whats the best way to kill German cockroaches?

Try sprinkling some borax powder in the area you see them. Don't leave Borax where children or pets can get at it and don't breathe it when you sprinkle. Borax is a laundry aid you can find in the laundry section of the supermarket.

go with a roach bait that comes in the large syringe..professionally it is what I mainly use for German roach issues. Combat is a brand you can buy at a store which is made by the same company that makes MaxForce sold to professionals.

Call Billy the Exterminator

Combat gel bait will kill the German roaches. The roaches eat the bait and take it to the dark places where they hang and this gets it to the roaches there and they will die.

Tell them to invade Russia.

Tell them a joke, and while they all stand there completely puzzled...

Stomp em!

Incinerators

Yeah that's bad...