First of all, You will probably need 150k score to get a S.
So, how hard to get a S?
Combo:
http://i.imgur.com/Gvcj4ZP.png
The score is to the quad power of the average combo you made. You'll probably want a score of 90k~100k here to get an S (with a normal team), meaning your average combo should be above 8.2.
Turn:
http://i.imgur.com/iKp5YKT.png
You have a base score of 65k and get a 2.5k deduction each turn. On a 10-floor dungeon, maximum score you can get here is 40k, when your echidna is just ready. When your max skilled gravity is ready, you can only score 27.5k from this section. So I assume you can get ~30k from this section on a average dungeon (echidna ready + 3 turns to kill, or OSK with Chinese god skills).
Skill boost CAN earn you some score here!
Rarity
http://i.imgur.com/xI9zkkW.png
Not exact. Might be off by ~100. The power should be 4.00267-ish.
Consider this is a Mystical dungeon. On average, you team should have two to three R6 gods including the helper, one or two R7 gods from descends, and maybe one ready-to-ultimate-evolve R5 subs. That makes your Rarity score well below 20k. There are also special teams that lower the rarity just to aim at getting ~100k here for an S.
So,
With average burst team that have rarity of 35+, waiting for Echidna, enhance-rin, or orb changers (12turn+), you'll still an average combo of 8.5 to get that fricking Tamadra.