Great Sword has the most powerful attack in the game. That's kinda it's thing, slow but powerful. If you noticed while using it that you can charge it up, it has 3 charges, if you time it right at the right time after the third charge you'll get the most powerful attack in the game.
With the Great Sword your going to want to focus on the Draw version of that charge attack. If you just do the combo attack with it, you're "doing it wrong". Focus on getting in the right position, with your sword sheathed, then start the charge attack and time it perfectly and you will be doing massive damage.
The Great Sword is a ton of fun when you learn the monsters patterns and can time that perfect triple charge draw attack.
Don't just go swinging away randomly with it, that'll get you nowhere. Always use the charge attack. Even a single charge or double charge will be good, but try to get the triple charge and release at the correct time for the 'super charge'. It's kinda like figuring out the boost at the start of Mario Kart games to get that super charge. Their are visual cues though and you'll hear a sound if you get it.
Watch this at 15 seconds in:
https://www.youtube.com/watch?v=Hev6gXAyWZs