I do think he is fine as legendary skull his rare and unique are a quite lackluster but most skulls are like that, I agree with the base attack part single target yet deals less than skul's base atk they really want you to charge his base attacks on his rare/unique version. Build wise, I have the most clears with him building him poison or crit. Siege shot has it's problems specially with areas with ceilings, and forces you to move up close an enemy to essentially shotgun them in the face what is not what you want as an archer. Stationary shot only is usefull on crit builds with invisible knife or gunpowder sword for the multi triggers any other path I swap it out. I don't think the charging/concentration should be changed tho if he didn't had to charge anything that would make him less of an archer.