The Grand User
Explorer
Sadly in their quest to simplify things WotC abandoned the practice from 3E that all creatures are build by the same rules and used a more video game design were everything not a PC, with some exceptions, are crated by a completely different set of rules or just rather arbitrary guidelines.
Sadly that means that playing monsters or anything WotC didn't deem to be appropriate rather hard. Better switch to 3E for that.
I actually don't have a problem with building regular PCs and monster PCs differently, as the goal isn't to build things the same way, but to have a comparable level of power after they're built. And even in 3E the only thing that was the same between PCs and monsters was that HD and the number calculations were the same, but not what abilities were appropriate.
I could see making a PC with using a monster stats as a base and adding some class abilities to fill it out. Maybe lower the HD a bit, apply the same Proficiency bonus as appropriate for the level you're aiming for, and a few other tweaks like that, and it should be easily playable with further levels from the class as normal.