As a GM, I came up with a simple mechanical solution.
With more than one person cooperating, I take the highest attribute on a side and add half of the second highest contributor's score. So in the example, player A with a 30 score still wins, because he is facing 20 + (15 X .5). So it is 30 vs 27.5.
With each additional contributor, I half it again. Say the 30 warfare person was fighting a 20, a 15 and a 12. Now the 30 *barely* loses since he is facing 20 +7.5 + 3= 30.5.
Of course it can get complicated when the players change from a warfare contest to another attribute mid-fight. But that's part of the fun.