Giantdad is easy to deal with till you get hit. After the first hit on you, you're probably done for - that's how it's designed.
Dark magic is good, if you're not one of the 2honorable sorts who doesn't keep it equipped.
WoG too.
For more ~honourable~ magicks, slow paced stuff will actually do well - giantdad isn't the best at dodging, and can't do shit for blocking. Emit force, gravelord (great)sword dance, fire whips, firestorm, white dragon breath. Not a lot of stuff you're likely to have equipped, but general ranged spam also works.
You could poison them and run away, or attack when they try to cure. Poison probably won't do much to them though - they have a lot of health, so they're likely to ignore it.
Light weapons and rolling are going to get you screwed, unless you're exceptionally good at rolling.
That said, rolling is going to be better than almost any shield. I haven't seen how greatshields hold up, but normal shields will get knocked over with giantdad stamina to spare.
It's a zweihander, so parrying is a definite option; but if you miss you're dead, and the giantdad won't go down with one riposte.
Overall, spam shit and stay away from that goddamn blade.
As majikero said, pyromancy is good for that. I'd recommend fire whip over combustion personally, due to its greater range, but fire whip may not break the giantdad's poise so idk.