I'm sorry, let me clarify, my advice is to help you get through the Black Omen. Also, what is your party equipped with? Have you gotten the Moon Armor or Prism helms yet? They will boost your defense greatly. The only attacks Lavos have that kick total ass are "Destruction Rains from the Heavens" (1st Form) and Grand Stone(Third Form).

It sounds to me like you may have skipped a few "optional" quests.

As for Completion in VII. Max Levels, all equipment and items, all materia types (including Master), All Enemy Skills, Defeat Both WEAPONS, and all Limit breaks denotes 100% Completion in my book. If you are really anal, I guess you need one Mastered version of every materia type but I feel this is a waste of time in the long run.