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I dont know what the OP is talking about, i made a 2H paladin and my starting party consists of Valeri, Amiri and Linzi. Playing on hard and every fight has been a cakewalk so far, most enemies die in one melee hit.

Pathfinder fixes a lot of flaws with 3.5 but still havent fixed the problem of low level casters being mediocre sadly. Tartuccio was useless during the tutorial, cantrips doing 1d3 damage on a ranged touch attack is just sad and 5 spells per day just doesnt cut it when you are fighting 5+ encounters in a dungeon without being able to rest.
I've done two complete playthroughs, once with a Rogue and once with a 2H Paladin. I started a playthrough with a Cleric, but didn't finish.
Interesting topic. I'll add my voice to those who played through the game as a melee character and didn't have a problem.

The point buy in Kingmaker and Wrath are both very generous. 16 before racial bonuses would be normal for your highest ability score, and the only class for which Constitution should be your highest score is the Kineticist. Paladins want Strength and Charisma, and only need a touch of Dex and Con while they're free to dump Wis and Int (though on your main character you're probably going to want a little bit of Int). My Paladin went in with 16 Str, 14 Dex, 12 Con, 12 Int, 7 Wis, and 16 Cha, and with racial bonuses from Angelkin Aasimar that puts you in a very nice position with an 18 in both your primary and secondary stat while still having passable intelligence. And even with 7 Wisdom a Paladin is still going to have great Will saves.

The idea that you "need" a dedicated spellcaster is just incorrect. Even in pen and paper something like a bard is perfectly sufficient. While a lot of people are aware that wizards are basically at the top of every Pathfinder pen and paper class tier list, there is often a great degree of misunderstanding as to why they're so high. While the wizard is no slouch in combat, the real power of the class is outside of the battlefield. The wizard's spells are open-ended, and if used intelligently can completely change the course of the story. Remember that section of the story where the trickster King is undermining your Kingdom with covert operations you just can't stop? What if you could do that kind of thing to totally undermine your enemies? Welcome to the pen and paper wizard, which can break the game in ten different ways simultaneously if they really want to.

Kingmaker and Wrath just don't support that same open-ended nature that pen and paper does. There is a limited decision space, and if the game is programmed that there will be a boss battle with exactly these enemies and you will enter through the main doors... well, that's the way it's going to happen. This robs the wizard of its main strength, the ability to say "actually, this is how it's going to go down," and take control of the narrative with his literally reality-warping powers. Moreover, many spells are either severely gimped or removed entirely. Almost every spell had its range severely reduced, and many others lost a great deal of utility or are just not in the game. Wizards in the Owlcat games are a shadow of their pen and paper incarnation.

The big issue here is that the game is just badly balanced regardless of what class you play. Now it made perfect sense that the difficulty would increase from the pen and paper games. You can't reload a saved game in tabletop play, and giving the ability to reload takes away a great deal of the risk so compensating with more difficult fights makes a lot of sense. Unfortunately the Owlcat team both overdid it in terms of how much they raised the difficulty, and the way they chose to raise the difficulty was not well thought out.

Owlcat primarily increased the difficulty by adding more AC, Attack, Damage, and Hit points to enemies. Now in moderation this is not a bad idea and can be a part of a larger plan to increase the difficulty. The problem is that they really leaned on this one mechanism for difficulty and pushed it way too far. When numbers get really high they completely invalidate certain strategies and require very carefully planned counterplay. Now this is kinda interesting for optional encounters or maybe a "challenge mode" for the most hardcore players, but your typical RPG player or even your typical Pathfinder veteran this is going to find it needlessly punishing when there's just a total roadblock standing in front of the main story or even a prominent side-quest. And there are other ways to increase the difficulty; throw more enemies into the encounter rather than just stronger ones, a larger variety of enemies to give more strategic depth, and unique environmental challenges to overcome. Big stats get boring really fast, and when it's on the main storyline it's utterly frustrating. It restricts the range of viable class builds (you need an AC stacker in the party, which is not the case in pen and paper at all) and means that critical hits are often instant kills because the base damage is so high.