I might not be the best source but from what my experiences are, (for now) the most powerful build is a spellblade thief, the tweaked one the person who's beat kass at winter palace multiple times, sorry forget his name, made. charge weapon, triple threat, sure shot, eviscerate, unbreakable, c petal and kunai (to apply bleed) then swap kunai for sanctified gladius so you get your passive activated and have more damage output from the gladius. unbreakable to ensure even if your entire party is nuked you live, sennenbari belt for a second chance, and just enough time to do your combo ensures at least the possibility to near one shot anything, and the survivability to do so. just minimal aoe, but besides kass party should be able to do that. ag str and cunning, with drinks if needed. you can also swap around weapons if needed to exploit weaknesses, which circles to original question.
imo mages primary issue is that generally their spells are a wee bit lacking, as the game progresses though i'm sure they'll begin to be more competetive as their newer summons, the f knight and dancer, are quite quite good, at least when not comparing the kiyoko summon which is prettyyyy solid. as enemies have gotten more extreme resistances, mages require both more gameplay knowledge and the spell pool. a current high power boss is immune to fire for example. thieves and warriors dont seem to have as much a issue because, generally speaking, defenses or not nearly everything can be beat to death. mages are kinda closer to final fantasy style where they are very good but they need at minimum a high power spell of each element, and as it stands there are spells of each element but of vastly different power levels. some spells like magic missile are neutral and powerful but take a bit too much time to build for fights that'd count currently. imho no class is particularly weak right now, i'd probably just build a class around your favorite party members or favorite items. even white mages have some fun stuff appearing like the new fire jade, i think its called unique off hand that boosts healing spell power. tanks got a cool unique shield from the salamander shop in khor minos, warriors now have a additional passive to support dps with 2 handers or no shields, charmers have lots of nuke potential and good party synergy with quinn and horny azyrran. as new class kits come out that'll be less important also and could probably make a decent team regardless.
unless you're doing ironman or low level runs i really would just recommend having fun however, you can min max a lot easier now also with affinity alterations also, and above all else nothing matters if youre not having fun or enjoying your rp potential, if thieves were op but you like shields, if mages were op but you like melee etc, what matters is what you enjoy, same with party members. atugia's solid but azyrran and arona are definitely good enough and have bit more damage output, main thing is looking at their kits, and say you take atugia, you probably want your healer to heal less and damage more to compensate, or if you take etheryn as a scout, same thing. you want a good balance of the holy trinity of attack defense and healing, thats the important bit. too tanky, you dont clear fast enough or last too long, too much healing or damage same thing. of course though that generally only matters at bosses. i really enjoy min maxing and stuff myself but i'd definitely recommend prioritizing fun, if you truly enjoy the abilities or rp or what have you of any class they can all pull off harder content with enough effort and preparation.
