Nioh 2 has a bunch of interlocking stats and systems that can give you a much-needed edge in combat, and one of them, in particular, is worth shouting out: you gotta stay on top of your Soul Cores. It took me way too long before I realized why my Guardian Spirit felt so weak compared to the first game.
Building off what Chris already said about activating Yokai Shift (your powered-up invincibility state) liberally, I wanted to add that you can boost your Guardian Spirit's attack and defense with Soul Cores, something that's easily overlooked for new players and easily forgotten if you don't form a habit. Soul Cores are new-to-Nioh 2 items that let you morph into enemies (!) using your purple Anima energy bar.
The basic loop goes like this: play Nioh 2 as you normally would, pick up any Soul Cores that enemies drop, bring them back to a Shrine (or just finish a level) to purify them, and open up the Manage Soul Cores option from the Shrine menu (also accessible from the Starting Point icon on the map screen).
Put another way, just get to the menu where you level up, and scroll to the left two spots.
Select "Attune Soul Core," then check out the Cores' varying levels, stats, and perks (there are video clips for each Soul Core attack) to figure out what looks best for your build and playstyle. Now, you're going to want to equip ("attune") as many Soul Cores as you can to your primary Guardian Spirit.
Initially, you can only have two Soul Core attacks – R2 + Triangle and R2 + Square – but further into Nioh 2, you'll be able to equip and use a third Soul Core attack in combat with R2 + X. Here's an example:
Without these Soul Cores, my Guardian's attack and defense rating would be far lower. If you don't regularly rotate in higher-level Soul Cores like you would traditional gear, you might run into problems.
There are a few other things to stay on top of, too. You can power up your Soul Cores' special passive effects by sacrificing lower-level Soul Cores of the same type. From the Manage Soul Cores submenu, pick Soul Fusion. Next, select your "base" Soul Core, then a lower-level throwaway "material" Soul Core, and pick "Perform Soul Fusion." Try to keep doing this until you max out the Soul Core Rank (at nine).
You can also recycle extra Soul Cores that you never intend to use. Doing so can level up your Shiftling (Yokai Shift) skill tree – which has crucial bonuses – and earn rare crafting materials for the blacksmith. This process is known as Resting Rites, and it's accessible from the same Manage Soul Cores submenu.
Nioh 2tells you all of this information, but if you're anything like me, you'll just dabble and then file it away on your to-do list. Don't be like me! Stay on top of your Soul Cores. They're hugely beneficial.
If there's one Nioh 2suggestion I want to share above all others, it's to use the Options (start) button when you're digging around in menus. If you press Options and scroll with the d-pad, you'll see detailed explanations for every single stat, ability, perk, and mechanic in plain English. It's a godsend.
As for which Soul Cores you should use, the jury is still out, but I love the Yoki Soul Core against human enemies (you'll do a forward slash that can knock them down) and the Kappa Soul Core (for defense).