Generally speaking, these 'clear ultimate with just R kamihime' missions tend to be 'pick the element with advantage and hope you have a high enough atk and assault to punch your way through'. And a lot of the time, it also implies that your soul will be using Sniper Shot.
Against Medusa, I think that it's Dike who has a def debuff to help you punch harder. If you're really worried about an instance of petrification, you'll probably have to depend on your soul for it. Or, go off element and slip in Indra for her cleanse if you're confident in not needing another light kamihime in there. Or, having a friend with Behemoth would take care of that with no downside.
As for Rahab? No current thunder R has a debuff that I know of. Consider putting in Perun for her damage cut to help survive 1 raging overdrive. Beyond that, see the 'have a high enough atk and assault to punch your way through' part. Which is what thunder currently does anyway :P
As far as example atk/assault/eidolon setups I did those missions with?
For Medusa, my kamihime had ~31-32k atk, 62% assault from weapon grid, +25% assault from Sphinx (friend), and +45% light from St. Nicholas (mine). Andromeda w/ Sniper Shot as my soul. I think Andromeda was the choice to make sure I come into the Medusa fight at full HP. I could not full burst on turn 1, so I got 4/5 of my team petrified. But Sniper Shot, Urania's atk debuff, and raw HP allowed me to just wait out the petrification to finally burst. Did have to eat 1 raging overdrive. Yay having sufficient stats to not care. But seriously, if possible, a Behemoth buddy is preferred :P
For Rahab, my kamihime had ~30-31k atk, 60% assault from weapon grid, +40% assault from Yggdrasil (mine), and +55% thunder from Thunderbird (friend). Cassiopeia w/ Sniper Shot as my soul. Ate 1 raging overdrive.