For this intensity I would suggest AMS2431/4B AWS 28. However, you also need to be sure this size media is capable of getting into the smallest radius of the part. Media as small as AWS 20 could probably achieve a 12A depending on the media propulsion method. However, propelling media at the necessary velocity to achieve at 12A is not recommended. You run the risk of abusing the surface rather than peening it.
AMS2430 REV T
3.6.2 For fillet radii on parts, the shot size used shall be such that the shot nominal diameter is not greater than one-half the smallest fillet radius to be peened, except that the nominal diameter of steel shot need not be smaller than 0.007 inch (0.18 mm) and the nominal diameter of glass shot need not be smaller than 0.002 inch (0.05 mm). If this requires a shot size that is smaller than that shown in 8.4.3, the fillet radius shall be peened in a separate setup subsequent to the one required for general peening of the part. An intensity for the fillet radius, compatible with the smaller shot size, shall be established by the cognizant engineering organization. No additional masking of previously peened areas is required.