When you're picking your sacrifice groups we decided that on 3rd banish we would chuck in a tank, 1 healer and the worst DPS for execute phase. 4th banish the 2nd tank and the last 2 healers went in. On the basis that both your tanks are dead and the boss will be running around 1 shotting people anyway, healers are useless here, the boss needs to die now and you can't heal through it.