Max concentrated shot disgusts me. About 5 shots dealing mediocre damage and a long duration? No thanks. I wouldn't max it for defense in stages, because lvl1 does perfectly fine against, and any level concentrated shot is nearly useless against OP unflinchable bosses. It's also poor in offense, since it takes three seconds for just 600~800 dmg. If I wanted another dmg skill, I would max Flame Impact because it's faster, cancellable, and a hkd, which works wonders against mobs.
HP regen isn't necessary, but it's useful if you plan on soloing or staging without any healers. You're actually perfectly fine with just iron bomb, the railguns, your rb skills, and your slide shots. The rest is just icing on the cake.