Dante Rebellion Sparda
7 is pretty easy without summons.
Here are a few of 7's better known challenges - and they make "no summons" totally casual:
IENANENPANLBCMO
Initial equipment, no accessories, no escaping, no physical attacks, no limit breaks, command materia only (and at that, no command materia that cause physical attacks.) Challenge starts outside of Midgar.
SCIENMNANE
Solo character (except where another is mandatory,) initial equip, no materia, no accessories, no escaping.
SCNIIENANE trades the "no materia" restriction for "no items" and, for obvious reasons, bans Throw, Steal, Morph, and W-Item. Batteries (mandatory) and Save Crystal are allowed.
LLNIIENACMO
Low level (less than 30,) no items, initial equip, no accessories, command materia only. This is actually an evolved form of a previous challenge that didn't use "command materia only." This version allows Deathblow and Double Cut, unlike the other... while also requiring frequent party-member deaths that cannot be easily reversed... because less than 30 is an absolute rule. If one character achieves level 30, you lose.