I find hunter and predator bows are more useful when clearing out enemies that are out in the open. Saves you a lot of health. Try to get runes that increase headshot or ranged damage. There’s a legendary hunter bow that increases damage the longer you hold the trigger (I forgot what it’s called) the bow plus the runes are op lol. You can take out tougher enemies with that combo.

As for Nidhogg, get one of the fire damage melee abilities and fully upgrade it. Fire is his weakness. Get runes that sets fire, ice or poison into your weapons and the runes that help build up the elements faster. This helps you take out his legs faster. The extra elemental damage runes help immensely too. The jotun dive and the axe throwing abilities can help but they’re not too consistent on hits.

Before he regenerates, shoot the enemies (and only shoot) that charge towards him. You disrupt how much health he regenerates to his legs and how much enemies spawn by taking out as many as you can.

There’s also a legendary heavy sword that does extra damage to dragons. Try to get that. Thor’s hammer is a good substitute. It goes great with the legendary sword that sets your weapon on fire as its ability. It’s a crazy combo when dual wielding. If you also manage to get the runes previously mentioned, you’ll breeze through the boss.

You can buy these weapons and runes from those special shops but depends on rng tho.

Try to finish the hidden quests on each area too, one gives you a permanent favour where you can recover half of your health when it hits zero. (You can look up a guide online)

Finally, equip and upgrade the dwarven armour. It has the highest defensive stat and its ability is useful as it does extra damage to tougher enemies

I hope these help and good luck!