Pirate boss is too difficult, but why?

You definitely don't need to trade to get these upgrades. I understand, I also despise trading. For flasks, you can get Instilling Orbs from Faustus's Currency Exchange. They're about 20 Instilling Orbs for 1 Chaos Orb. These can go on your utility flasks and give them implicit modifiers like "used when charges are full" or "used when you are frozen" etc. They make it so you don't have to spend all your time pressing flasks like some kind of demented piano player lol. You may want to look into alternate healing sources like life leech, regen, and Enduring Cry. Enduring Cry is basically a very strong life flask using a skill gem slot.

Unfortunately, I don't think you're going to be able to make crits do more damage than Resolute Technique without trading. Resolute Technique is very good and it saves you a lot of passive points and gear mods that you would have to use on accuracy. Maybe focus on doing some Abysses for those Stygian Vises, jewels, and the high value, low risk XP that abyss monsters bring. I'm assuming from looking at your character that abyss monsters will bounce off you like hitting a brick wall, for the most part. There are jewels that will make a mana flask completely unnecessary, and there's a good chance you'll find one if you do abysses.

I might lean completely into the Heavy Strike and get the Tribal Fury node. If you get that and the mastery node that adds an additional strike, I think you'll do more damage and have better AOE than ground slam. Then you can use the new gem slots for something else, maybe Herald of Purity, or Pride, or something else.

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Unfortunately, I don't think you're going to be able to make crits do more damage than Resolute Technique without trading. Resolute Technique is very good and it saves you a lot of passive points and gear mods that you would have to use on accuracy.

Yes, that's the conclusion I reached after struggling to align all the stars in the past. I'm better off with only one point spent, which turns a melee fighter into some sort of spellcaster, with 100% chance to hit against anyone, independent of your accuracy and their evasion rating. The rest of the points can be better spent elsewhere, on flat dmg and survivability. Same goes for blocking vs evasion. Nowadays it's much more expensive to have both attack and spell evasion and your attack evasion gets reduced by the enemy's accuracy. I use Versatile Combatant to save a lot of points that I'd invest to raise attack and spell block separately, even if my max block is reduced by 10%. This last issue is solved in the best way by the Gladiator class, with the lucky block chance. And furthermore by the shield I'm wearing.

EDIT: I don't think getting an additional enemy to be hit by heavy strike will help, because I only use it against single enemies, like bosses. Against more than one enemy I use the slam attack.

The Tribal Fury node gives strikes splash damage. When you combine this with the extra attacks, it becomes very crazy. You can test out the extra attack part by buying an Ancestral Call Support gem from Lilly Roth in town. It's quite strong.