Nomads. Each nomad is DS 8. Add 2 to the DS for each additional nomad in a group – four nomads (the most that reasonably can attack a goon) would be DS 14. For every 4 damage done to a group, one nomad drops back and the group DS gets recalculated. So the first 4 damage done to a group of 4 nomads would result in a group of 3 with DS 12. Nomads who fall back recover and come back the next day at full DS.
I would do the same +2 per opponent for any base DS, so four opponents individually DS 12 would be DS 18 against a single goon.
I would do 1/2 DS in damage makes a member of the opposing team drop back, so for opponents of DS 12 you would need to inflict 6 damage in order to reduce a group of four to three and drop the DS from 18 to 16.