Nothing annoys me more about the challenges than that fucker right there and the 'no awoken skills' modifier. I did finally beat challenge 7 and then 8 was easier for me. I don't know if I will even try 9.
With 20 to 30 stones you should easily get your shiva finished and get yomigon done. Probably okuni as well. I guess that depends on what your sta bar is at full.
During 10x descends you should be able to do that without any serious problem provided you are willing to stone for sta. I think I ended up +297ing 3 of the cards on my ra dragon team that weren't already maxed.
I farmed Zeus/Hera yesterday and managed to clear it 12 times out of 16 attempts or so. I am not looking forward to the skill up process of that card. Especially since its part of my strategy to clear Machine Hera multiplayer.
It wasn't till I went to ARa/Ra dragon my combo skills improved. Most teams you really only have to concentrate on making sure the lskill is used to clear stuff.