Grey Worm slits the throat of a Lannister guard, after sneaking past the walls of Casterly Rock via the sewer tunnels. (Kill of Unnamed Character)
It is a little difficult to differentiate Grey Worm from the rest of the Unsullied, but he has a badge with three dragons on the clavicle of his armor which seems to be unique to him, which was used to identify him in the battle scenes: http://imgur.com/a/BGogM
This first kill is the only ambiguous one as the clavicle piece is not visible, but it seems to credibly be him based on the parts of face that are visible.-nyan
The shield kills that are being counting aren't confirmed kills. Minor injuries at best. -talindsey
In the rulebook for Kills.IV we have a rule for: "Pretty much any kind of strike on random extras is counted for a kill unless the extra is shown to be explicitly alive after. This is applied often for large battle scenes."
It's just a lot easier to count, else there are a lot of nitpicky debates that some strike is on a non-fatal region, or the shield bash didn't look strong enough, etc.-nyan