Altering damage from client side is visually possible. By increasing your damage in your client files, you may see that your damage has indeed increased however it takes no effect on the mobs you're hitting. It's because damage values are given by the server and they're stored right in there, not in your client files obviously. The only way to alter the damage not only visually involves in hacking the server which hosts. I don't think anyone here could pull it off!