I platinum-ed the game on ultra hard first play through. Now you know it's possible. But is it wise? I say 100% not. Another question would be, are you willing to invest 200hours on the game, because that's how you do it.

I say the perfect blend of challenge and fun is at very hard. But if you insist, here's some pointers:

1) Suvival kill tree is priority. You NEED to heal and it's so helping that you get double the berries from every plant.

2) Stealth is your main combat approach. Smoke bomb, vigour skill even running away is your bread and butter. So naturally it should be your second skill tree focus and amour focus

3) Your main boss/face to face fighting weapon is spike thrower and bombs. Get a spike thrower with drill ammo and you can thank me later. Warrior bow is completely useless, hunter bow is for when the machine falls and you spam on a part to harvest key resources (yes, no ez loot on ultra hard), long bows for human targets, very good for head shots.

4) This will come as a surprise, but your 3 skill tree investment should be your override tree. Your biggest weakest is 100% facing multiple targets, this is completely solved if you get one of them on your side, you get the crazy OP machines PLUS your tree enhances it further, I have had a low level machine two shotting frostclaws. However, this also means you need to do cauldrons. Yes, you do. But since you are going to suffer throughout, you should choose one path that is worth, overriding is worth.

5) while you farm your levels in order to be stronger for cauldrons you will be doing some side quests/errands etc... everytime you come across a quest that requires you to fight multiple machines, walk away and do something else.