Stealth is a buff for you, so you should have checked off 2 on blargney's list above.
If debuffing enemies is supposed to happen via crippling sneak attacks and such, then Assassin is definitely your prestige class of choice. As for the Stealth, you can do that at low levels with mundane skill checks well enough, but it doesn't last long. By about 5th level, you'll want to have invisibility available to you regularly. It's not for the approach; it's for the getaway. Your Stealth checks have a -10 penalty after you attack (and that assumes you can even Bluff them off of you in the first place, not to mention have a hidden place to go and get re-Stealthed in; if you don't have both, you aren't allowed to re-Stealth at all). That's why the Hide In Plain Sight class ability of Rangers and Assassins exists (note, too, that it's very high level). Invisibility is much simpler- just 5-foot step away, drink a potion, and voila- poof, disappeared.
This is where it might be useful to go Vivisectionist Alchemist- they get it as an Extract, and besides that can make Potions to keep it around when you run out of Extract slots for the day. You don't
have to go Feral Mutagen and natural weapons to make Vivisectionist work; that's just the way most CharOp types prefer to do it since you get multiple attacks. Alchemists aren't normally proficient with Greataxes or Falchions, but Half-Orcs are, so doing a Half-Orc Vivisectionist could work (especially since Half-Orcs don't lose INT in PF). If you really like the Greataxe and/or Falchion, of course (both have their merits- and are superior to natural weapons for crits).