Which of these should I get? Watcher Greatsword or Watcher Shield? Does it mean anything by "Unleash it's true power to enwreath in magic."? Also why is it so difficult to parry in this game? I'm talking pve, in DS1 it's about timing and it's easy, but in 2 it seems to be based on luck.

If you're a faith build, get the Watcher Greatsword. It has an in-built lightning buff, which I guess is its "true power" so it's decent for any build, but it's exceptional when combined with faith. The Watcher Shield is really good for PvP, but not so much for PvE. It'll take your opponent's weapon durability down slightly every time they hit the shield, I'm not sure it's enough the make any real difference though. If you infuse it with lightning, it gets 100% lightning resist, and it has a high physical damage reduction too, so it's effectively a counter-faith shield. [b]It seems I got mixed up between the Defender's Greatsword and Watcher's Greatsword. Everything I said still stands though, but with an intelligence build rather than a faith build. Also, the Watcher's Shield doesn't drain durability, it's just a cosmetic effect.[/b] Parrying seem really inconsistent because they messed with the parry frames for different pieces of equipment. The closest item I've found to Dark Souls 1 parrying is the parrying dagger, and I'd recommend trying it out if you're used to the Dark Souls 1 parrying style.

Defender's Greatsword is for faith, not Watcher's. Also, Watcher's Shield does not damage durability. Honestly, I'd just use a good medium shield for versatility, both parries and good blocking.