The 3 main story Primals, in their weakest incarnation, were canonically soloed. Their HM versions, the other 4 story Primals, as well as all the EX versions...told to get 7 others to aid you...and all 7 HM Primal fights are cannon for 2.5 (Garuda, Titan, Ifrit, Shiva, Levi, Ramuh, Moggle). Operation Archon, Coil and the Steps of Faith all require 8 as per cannon (told to get elite adventurers for them). Heck, all of CT was gather TWENTY THREE others to go with you. And if you want side story...all the Hildi fights have you and 7 others as cannon.