When I was at a low level, I soloed mediums and smalls till about level 50, then did tried to do the last 5 floors I could do as c6 large 5:5 with a team. And you just find teams in the dung lobby until you are good enough to be in a clan. A lot of the clans aren't that active anyway so often better off just trying to get a team in the lobby. Once you can do floor 35, I would do every floor after floor 35 at least until you are around 90, at which point maybe every floor after 38. As you get higher you will want to rush (c1 small 4/5 person team) the lower floors, until the large floors as mentioned above. Once you have done all floors you should reset and start from the bottom again. Its a daunting skill to begin with, but honestly its so good after sort of 80/85+. I think its the best skill in RS, and I loved getting 99 in it. I have to stop myself from going back so I meet my other goals :)