Combining the best Hexes, solid natural defence and poise plus the fantastic move set of a particular boss' soul greatsword: this build is unstoppable.
Lightning Defender's Greatsword +10 - Unbuffed, this has an attack rating of 541 with this build, enough to tear through any armour. But then you add in the buffed (2hdL2) bonus damage, and suddenly you have a lot of burst damage. Its 2hd attacks are mighty fast with good horizontal arcs, too.
Lightning Disc Chime +10 - This will be your chime for this build. With it, you will have 511 attack rating, and GRS will hit for anywhere from 400-700 damage,with a decent cast speed (180). More than enough to stagger opponents - or end a fleeing opponent's life :D
Lightning Urns x99 - Vital to your strategy, and to keep opponents guessing. These types of items are very under utilised in PvP, so you have the element of surprise, too. They will hit for anywhere from 100-300 damage, but it is their stagger ability and surprise element you need to take advantage of with your spells… read on!
Kings Crown - +3 to INT and FTH plus a bonus to your cast speed makes this a must have. Bonus Fashion Souls points, too.
Dark Armour - Decent defence, poise and looks all for a low weight. But really, pick whatever you like for here.
Engraved Gauntlets - I'm not sure I've ever reaped the potential bonus damage these offer, but they look good!
Lion Mage Skirt - you need to keep your weight down, and the bonus to your cast speed will never go astray.
Third Dragon Ring - everyone's using it, so get on board.
Ring of Prayer - can be swapped out for Ring of Blades if you're that way inclined :)
Clear Bluestone Ring +2 - vital to get Force out as quick as possible.
Southern Ritual Band +2 - you need this one for the slots.
Great Resonant Soul - You'll only have 8 of them to fire off, but you'll rarely need more than 2.
Force - The bread and butter of this build. Again, woefully under used in PvP so it surprises everyone. Keep those Twinblade/Santier's suckers on their toes with this.
Great Magic Barrier - You can never have too much defence. Cast this upon entering the arena.
With this build, you'll be packing well over 500 attack rating in both hands, with over 1500HP, 139 stamina, ~79 equip load to play Fashion Souls with, 97 Agility to avoid all but the worst fire Katana phantom swings, nearly 25 poise (not that it matters much in DS2) and a super high buffed defence that can take a (lag infested) beating. I feel it could use a few more points of stamina, and I would probably give up a few HP for them, but this is mighty balanced as it is.
If you see a turtler, those Lightning Urns do a wonderful job of draining stamina and staggering. More importantly, they force rolls one way or the other, which you can punish very easily with Force or GRS. This is your mixup strategy.
If they're a mage or similar, you can go toe-to-toe with GRS, however their squishiness stands no chance next to the lightning infused Defender's Greatsword, so try to get in there. Again, lightning urns plus Force make this pretty easy.
Essentially, there are zero builds at SL125 (and hopefully FROM do something to make that a worthwhile statement!) you can't match. Havel armour you tear through with both GRS and the buffed sword. Nimble twinblade users you can blow up with Force, before punishing with your preferred follow up. People who love to run away and Estus in the middle of a PvP fight can get a GRS or a Lightning Urn in their back. Other mages or hexers are far too squishy to withstand a Force + sword onslaught.
I've tried the Blue Flame sword build, tried a pure poison build (Manslayer and Scorpion Stinger with Moth, Rat, Abyss and Dark Fog - great fun but no burst damage), tried a pure mage (MLGS and Wisdom), as well as Black Knight Greatsword builds. None of them offer the versatility of this build, and they all have compromises I couldn't live with. If your thumbs are up to it, you'll rule Iron Keep and Dragon Aerie no worries - just watch those wusses with their phantom fire Katana range :D