Originally Posted by
Laurcus
+15 Is going to be way better than +5 lightning. With all you've invested in str/vit/end I wouldn't go for int or faith ever unless you're hitting like level 250+ in a higher NG. Pyromancy is a zero stat investment, and while not as powerful as sorcery, it will do just fine and you don't have to dip away from your core stats for it.
One thing you need is more endurance. Get at least 40, and replace that ring of steel protection with the ring of favor and protection. Since you're past Anor Londo you shouldn't have any issues getting it. Endurance is the god stat in Dark Souls. It provides dps by letting you swing more, and survivability by letting you run/block/roll more. It really is the god tier stat until at least 40 so that you cap your stamina. After 40, it's still not bad if you need some more to get the fast roll with your preferred gear. I'd also grab some better armor if I were you. With some endurance stacking and the ring of favor and protection, you should be able to wear the giant's armor and keep your fast roll.
Another thing I'd ask you is, do you enjoy how the Zweihander plays? Fyi, except in very specific circumstances where you need max dps and don't care about getting hit, you generally want to only use jump attacks with most two handers. They're faster, more damaging and overall safer than regular attacks. If you like doing regular attacks, fast one handers like the Balder Side Sword are generally better. A +5 BKGS is 5 more base damage than a +15 Zweihander, but the BKGS has better strength scaling.
Lightning and fire weapons are fixed damage weapons. That means a +5 lightning Zweihander does the same damage at 25 strength as it does at 99. I wouldn't recommend going for lightning weapons unless your stats are really bad, which they aren't. Faith and int based weapons like divine and enchanted are really bad unless you're stacked on multiple stats, like strength and int for an enchanted Zweihander. That means being very high level, or sacrificing survivability. A buffed normal weapon, (buffs as in resin) will almost always beat out any other kind of weapon on raw damage.