IMO, scathing in your sentence means to be harshly critical of, bitterly denunciatory of, and by no means skeptical of.
I wouldn't use it together with 'a little bit more' since scathing has a sharp, corrosive ring to it, it carries a lot of negative weight itself. It is like saying 'be a little bit more ferocious to..'.
a scathing remark, scathing criticism / point of view / attack on... - harsh, bitter...
to scathe means - to do harm to; to injure; to damage; to waste; to destroy.
So, I would go for b since ruthless is closer to scathing.