Are you having trouble with the boss or with the level as a whole? The twinblade is a bit weak in the beginning but once you level your way upto 40 dex and start buffing it with resins or spells it becomes a monster. For now if you want a way to cheese past the cathedral try upgrading a regular longsword. It legit is the best weapon in the game(although boring), and you can clear the whole game with it. With your current stats a +3 longsword wil do 190 damage as compared to the 172 your getting out of the twin blade.

If you wanted to use zwei you shouldnt have levelled dex so much. Only a strength build will get proper damage out of it. Again until you reinforce it to about +5 and you have about 30 strength, smaller weapons like the longsword will be more efficient to use.

If you dont have a longsword already, you can buy one from greirat the thief at firelink shrine.

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