Merc is definitely useful in fauzt, a3 doesn't die, and can help with clearing, especially abjurer.
As for getting merc to survive in scos, I think first step would be to put 90 resist gear on it, and some way of rapid healing, either via leech, or ones that cast spells that heal (bloodmage), or rely on necrolyte "enemies deal less damage" via miasma/force blast and helping with chilling enemies, slowerd targets deal less dps.
There already are cases of mercs surving scos if you guys look at some showcase videos, but it is mostly done through just very good clear speed, so that merc doesn't have time to take any damage. Then you can argue merc isn't even needed. Key here is to get merc survive on a build that clears this place at more moderate pace.