The Rocky-helmet has the effect of causing any Pokemon that attacks the holder with a Physical Move to lose 1/6 of their health. Source
A good Pokemon to use this is Ferrothorn. This is because he has the ability Iron-barbs which causes any Pokemon that attacks Ferrothorn with a Physical Move to lose 1/8 of their health. This Ability Stacks with Rocky-helmet, and if you pair this with Leech-seed he can do massive damage to physical Pokemon.
This is also a good item for any Wall to have, but make sure they have a way of healing themselves, if they do not Leftovers is probably better.