If you are a dex-heavy fencer using full leather gear, you’d hardly need pain barrier to begin with. The extra duration of pain barrier will mainly benefit high str Fencers. If you were really that concerned about knockdowns from weak monsters or whatever you want to call it, you would be going with a Dex Fencer to begin with, negating the need to spend 2 circles (sw3) to get 10 more seconds on Pain Barrier.
Yes, Concentrate with the str scaling will be fairly strong. We’re talking about 400~ total damage on Concentrate with 550+ str. But one major problem: Fencers are about having consistent dps by spamming Flanco/Composee. Unlike Doppels, Fencers lack multi-hit skills that takes advantage of Concentrate (8 hits from Sept, 4 from Lunge, 5 from Esquive). You are not going to benefit from Concentrate as much as you’d think.
So not only will you be less effective at the whole “not needing to manually dodge” thing because you are 100% reliant on pain barrier as a str heavy build, you are also not able to make effective use of the supposed benefit for going Sw3 (str scaling Concentrate). Would you not call this a subpar build in comparison to the Barb3 or HL3 variants?