I honestly found my Dark Souls NG+ playthrough to be easier than my original playthrough, despite the enemies having increased health and damage etc because I knew the tactics and strategies to defeat them well. Not even the Four Kings gave me any problems on my NG+ playthrough. It was kind of the opposite for me really, things I struggled with in my original playthrough I sailed through smoothly on my NG+ playthrough.
I didn't have much trouble with Sen's Fortress, I pretty much know the place off by heart but if you want some advice try using pyromancy, it helps a lot because you can take out those pesky snake people at range and then make your way through the swinging blades stress free. Also the boss doesn't have much of a fire resistance either from memory. Hell, pyromancy pretty much solves all your problems in Dark Souls, I didn't even bother trying to parry Gwyn, just circled him spamming combustion and great combustion etc and killed him very quickly.
Back when Diablo 3 was a pretty fresh game, Act 2 on Inferno was a very frustrating brick wall for the very under geared me. Not much fun at all.