I think you might need more than 20 Savage Orcs. A 30 man block,
IMO, is the minimum, as they are still rather squishy, even with toughness 4, and you want all the attacks you can get. Maybe make them Big Uns!

Also, a Shaman in the unit with Lucky Shrunken Head is nasty. 5+ ward saves on a big block? Yikes!