I just find the OP kinda funny because just yesterday I read another thread where someone else complained that Barrier and Guard are absolutely useless and make the game near impossible on higher difficulties. As someone who's doing a first blind playthrough on Hard without level-up / character build guides and managing just fine (most of the time. I have had some difficulty with boss-fights, but usually it's because I'm being incredibly dumb at times and using staves with wrong elemental damage) I think that's a bit of an exaggeration. And I have to say I find this claim a bit of an exaggeration as well - it's not like the game's a complete cake walk. Well, not on Hard difficulty anyway, IMO. Not until I see the first Nightmare Knight Enchanter vs +x level higher dragon solo anyway.
And yes, the Knight Enchanter is a bit bonkers and a bit OP (but please don't nerf her!), but that doesn't really make the whole Barrier system useless. And, like people have pointed out, optimised builds have existed in the previous games and they have always been OP because they're optimised. Part of the fun of beating Nightmare is fiddling around with your skills, party builds and tactics until you find something that works for your playstyle. It's a puzzle that turns Nightmare difficulty from a chore you suffer through for the bragging rights into an exiting and fun challenge. Of course if you do optimised builds on lower difficulties the game's going to be a cake walk because you've just made an already easy game even easier. But if that's the case, bump the difficulty up a bit.
And if everything else fails, turn on the frendly fire and have fun roasting your companions!