for a shorter way, but keyboard only : press C + M at the same time and type in "Deathberries". You will get hidden deathberries. To use them, click on the cat you would like to kill and then they will cease to exist
Longer way: become a medicine cat (or wait for a gathering) and head to the fireclan medicine den and grab deathberries. Find a source of water and click on it while holding the berries. You will then get hidden deathberries. Click on the cat you would like to kill and they will die