Wo Long: Fallen Dynasty's first boss, Zhang Liang, is a tricky customer, especially if you've never played Sekiro, and you're not particularly used to this game's parry-centric game style. He's super aggressive, hard to dodge, and when your fighting companion says he'll get the boss's attention so you can heal, he just…doesn't.
What's worse, Zhang Liang has a second phase where he eats some demon candy, goes real red, and grows a spiky arm that he's fond of swinging all over the place. There are certain things you can do to make this fight easier, such as grabbing the polearm podao weapon from the chest in the village elder's hut, or levelling up your wood virtue to get that little bit of extra health. Either way, here's my best strategy for beating Wo Long: Fallen Dynasty's first boss.
How to beat Zhang Liang's first phase
Zhang Liang comes at you swinging his big mace, and sadly, if you want this fight to be over quickly, you're going to have to deflect his attacks (B on keyboard and controller). This will shrink his Spirit Gauge (shown below boss health) and eventually cause Spirit Disruption, letting you perform a Fatal Strike (LMB + shift or Y on controller) which takes off a big chunk of his health. You'll know when he's open to a Fatal Strike, because he'll stop moving and a red dot will glow in his chest, Sekiro-style.
The fastest, though most difficult, way to reduce his Spirit Gauge is to deflect every attack, quickly hitting him in-between each with fast attacks (LMB or X on controller), and martial arts or spirit attacks when you have time. Zhang Liang is a boss that becomes infinitely harder if you don't deflect him on each initial attack, because he combos for days, and gives you little breathing room if you mess up. It's better to control the pace of the fight by deflecting those first attacks, and stun locking him with a few slashes after each to slow his momentum while chipping down his Spirit Gauge. Zhang Liang can actually be stun locked out of quite a few of his attacks.
That said, if you don't feel able to deflect all of his attacks, then you should at least aim to deflect the critical blows where he glows red. Deflecting these reduces his Spirit Gauge by a massive amount, and provided you are dealing some damage, should be enough to tip him into Spirit Disruption.
While most of his regular attacks aren't too hard to deflect, here are the ones you really need to watch out for:
Jumping mace slam
He jumps into the air with his mace and slams it down on you. The deflection timing is tricky as this attack massively speeds up at the end, so you should press the input while he's still in the air—do note this is different to the slower jumping slam where he goes high into the air later
Uppercut critical strike
He glows red and lowers his mace before swinging it in an uppercut. You should time the deflection for the instant his mace reaches the floor behind him, but often I just dodged this one since its range is limited and it hurts if it gets you.
Running critical strike
He glows red and readies his mace before sprinting at you. The trick to dealing with this attack is to not deflect as soon as he gets to you, since he delays a moment before actually swinging. Wait a moment when he arrives, then hit deflect.
After deflecting his critical strikes or regular attacks, you are safe to hit him until he gets up for the former, or starts blocking for the latter. For me using the polearm, Zhang Liang was often stun locked by attacks after a deflection, so it's well worth getting some hits in then.
How to beat Zhang Liang's second phase
Zhang Liang's second phase is a lot to deal with: he grows a big spiky arm that he tries to whip you with, and has a variety of annoying shockwave attacks, sending stones spiking towards you through the ground, or spiking the ground around him. The good news is that you only need to get him to half health, and you can do this by baiting out his red critical strikes. In particular you want to make sure you deflect the attack where he fires his spike arm at you, since this almost completely fills his Spirit Gauge, and he's usually stunned for long enough after that you can get a sneaky combo in to tip him over and perform a Fatal Strike.
Once he's at half health your companion will tell you to "release the power of the jade" and you need to press Y+B on controller. This will finish the fight early and save you some trouble. Zhang's red critical strikes seem pretty random to be honest; one time he did the spike arm launch right at the very beginning, whereas another time I had to hit him a bit before he did it. Either way, there's no point trying to get super aggressive with this phase and deal lots of damage conventionally—not when you can bait those critical strikes.
As in the first phase, there are a few attacks I'd suggest looking out for:
Spike arm critical strike
He glows red, then fires his spiky arm at you from a distance. This is the critical strike that you need to deflect to make this second phase less painful, since it does massive Spirit Gauge damage, and generally gives you enough time to combo that into a Spirit Disruption so you can stab him.
He spins his mace then comes charging at you from a distance. Like the jumping mace slam from the first phase, this is a very fast attack, so you'll want to deflect a little early in order to deal with it.
He plants the head of his mace against the ground then sends out spiking stones towards you. This can be deflected like any other attack and that's a much more reliable way to deal with it than dodging. After you've damaged him a bit, there's a chance he'll start doing two of these in a row, so don't relax after you've dealt with one.
The other critical strike to watch out for is when he charges up and comes running at you flailing with his mace—usually after you've already damaged him a bit. You can deflect this on any of the individual mace blows, but be warned: he'll often immediately follow up with another red attack firing his spike arm. If you can deflect both this is an immediate Spirit Disruption, but if you mess up on the spike arm, you'll get grabbed and he'll deal a lot of damage.