When you defeat the first Demon Wall a secret "path" is available and will lead to a treasure chest that contains the Demonsbane sword. Although, it may take a few tries and luck to actually get the sword. I'm pretty sure the first Demon Wall is around level 20 +. I'd suggest some leveling and once the battle starts begin with a good Quickening, have a party of heavy hitters to dish out damage and use items rather than magic for fast healing.
The only sword that has 140+ attack power is the greatsword Tournesol which can be purchased in the Bazaar after selling three Gemsteels, three Empyreal Soul and three Serpentarius; all of which come from high level monsters.
The Zodiac Spear may not be the best, but it is certainly far more easier to obtain than the Tournesol.
And I don't know what weapon your friend is talking about. The only weapon with +77 attack is the Sledgehammer, found in the Highwastes or Phon Coast, both of which are very far from where you are in the game.