Carter - I would first try a 5 to 1 ratio, and use a 3cc dart. That has always taken down my most difficult, but sometimes just barely. So because of the ''just barely part'', don't be afraid to use a 4 to 1 ratio and then use a 2cc dart if the 5 to 1 and 3cc dart don't work. Bet a dollar that the 4 to 1 will put his butt down, at least enough to sneek up and push another 1/2cc in if need be.
Know that there is a significant difference in knock down power between the 5 to 1 ratio and 6 to 1 ratio. But like I said, go to 4 to 1 if need be.
And it's probably just me, but it ''seems'' as though I have better luck in the dart's effectiveness when I put it in the neck/shoulder area. One definitely has the perpendicular shot presented quicker and more often in this region.
Good luck Carter!