As a Ratrbro, I am going help redeem my soul by spreading our secrets: in the Tomb of Pharros, expect an attack from above right after you enter. It is
the prime spot to strike you down, while you're distracted and confused. With the cat ring, they will take minimal damage as they drop down beside you and jump attack you from behind. If you're not moving (or even moving slowly in some cases) this attack
will hit you. If they're using a powerful weapon, this will be half of the battle over in a flash. Ratbros that are catbros end things quickly or not at all - the entrance area will make or break their success.
The next best location, to take down a Ratbro's prey is the ladder in the second part of the level. From here they have many options to attack or evade you in an advantageous position. If you're lightly armoured, the ladder will be very difficult to climb because I would expect most Ratbros attacking here to have a good ranged attack. They will be able to snipe you from above, so you will have few options if you want to progress. The best solution is to bait the Ratbro down to ground level and then hoof it upstairs. Ratbros that are ladderbros are going to be boring to fight, because they rely on a battle of attrition.
If you can survive both of these prime ambush points, you will almost certainly win against the Ratbro in one-on-one combat. Ratbros are, by their nature, weak and cowardly. They exist to ambush and end the fight as quickly as possible, because every second that passes evens out the battlefield a little more.
The key to victory is adaptability - if you rely solely on one attack you will make it more difficult to win. This is not one of the Belfries and being an efficient killer is not necessary. Ranged attacks, a shield, and a melee weapon are all recommended to fight against the Ratbros and their schemes, to survive against their attacks, and to endure their cowardice.
Adapt to survive and thrive, or let the rats win - which no one truly wants.
The only way to truly beat the Ratbros is to fight them... do not join the covenant to skip their domain, use this as a chance to beat them back. Anything less than absolute victory against the Ratbros is giving the Rat covenant what it wants - absolute domination.
Is there really any potential to power stance with two swords/ curved swords in pve, or just do it because you can?
I found dual-wielded curved swords to be rather useless, but I never invested in upgrading my offhand so that was probably my downfall. As a lightweight DEX build the off-hand powerstanced attacks took too long to use. In retrospect, if those attacks were stronger then I might've been able to stagger enemies more reliably, allowing for bigger combos. So, at least for me, it was better not to dual-wield...