Yeah, I'd invest in HP, since Armaldo already has great Defense. Investing more in it actually won't make a big difference.
I'd go like this:
252 Attack
252 HP
4 Defense / Speed
This makes your Armaldo into a tank, which means he can used as more than just a Stealth Rock Lead. You can switch him in, use his excellent STAB moves, and probably KO a couple Pokemon.