Grenades and melta bombs can be used normally against a walker if it is already immobilised at the start of the close combat. Otherwise, grenades and melta bombs may be used against a mobile walker using the Standard Rules for Krak Grenades and Melta Bombs, but must roll a 6 to hit.
Designer's Note: This is a new rule to give troops with krak grenades or melta bombs some chance to damage a walker – it is a poor chance but better than none at all.