I don't actually know the lowest possible EXP amount until Seph. I don't believe that you could get to him below 30 without doing the Level 1 Gameshark cheat. Level 43 is already considered low for beginning WtNW (even though Sephiroth is unlockable before then).

My lowest level was whatever level you get Second Chance in the Defender Path (probably 49 or 50).

The fast-track to beating Sephiroth is Reflect, not Berserk Charge, even if people say otherwise.