well for a SS its gotta be either regular smf, macv with a party, or just killing regular mobs on an appropriate level map.
Regular smf might be worth it still just because the xp of each goblin will be about the same as a normal level 70 mob- as long as you can reset it fast enough it would most likely be faster than a regular map...but regular maps can give faction fame and lost powers so that's a definite trade-off.
MACV would be ideal but you'd need a party. Some of the units in there are antistun so soloing it would be difficult.
And I know this post is old so hopefully you still see this but you need to be taking charms. I see sooooo many people working on mastery without charms. With a few charms you can work for an hour and complete four hours worth of work...I guess the few hundred gold isnt worth a few hours for some people.
At least get the guardian of saphael 10% bonus for helping out lowbies. Wear legacy and try to find some cheap charms- its soooo worth it.