Wo Long: Fallen Dynasty's deflect difficulty is a massively important stat for defeating difficult bosses and enemies, but it's not something the game really explains up front. Unlike Nioh, its spiritual predecessor, Wo Long involves far more attack deflection than the average Souls player is likely comfortable with.
If anything, it's comparable to Sekiro: you deflect a boss or enemy, filling a gauge that eventually lets you strike a fatal blow. That said, Wo Long does have some other systems such as morale and deflect difficulty that complicate things. If you're struggling with that first boss, Zhang Liang, hit the link for some tips to help you out. Otherwise, here's how deflect difficulty works in Wo Long.
What is deflect difficulty in Wo Long: Fallen Dynasty?
The deflect difficulty stat reflects how much spirit it costs to use a deflection or dodge against an enemy or boss. Spirit Gauge is the most important combat resource in Wo Long, allowing you to cast Wizardry Spells and perform Martial Arts. Vice versa, you'll need to disrupt a boss's spirit with deflections and attacks so you can perform a damaging Fatal Strike to lower their morale (and so too their difficulty).
Think of it like all-purpose stamina: if a boss hits your shield in Dark Souls and you don't have enough stamina to take the blow, you'll be stunned, and the same thing happens here—enemy attacks lower your spirit, and if it hits that bottom level, you'll be out of action for a brief period. This makes Wo Long's combat a balance between gaining spirit through attacks and deflections, then consuming that surplus to perform extra damage via Wizardry Spells or Martials Arts. Dodging also consumes spirit though has no chance to return it, unlike deflections, so is generally less preferable.
Different weapons have different deflect difficulty stats; the higher the percentage, the less it costs to dodge and deflect. Generally, the rule here is that bigger damage-dealing weapons like polearms and maces have lower deflect difficulty—meaning a higher spirit cost—while swords and smaller weapons have extremely high deflect difficulty, but lower damage. If you're someone who prefers to dance around an enemy slashing them to ribbons with numerous strikes, swords are definitely the way to go.
Levelling your Water Virtue also lowers the spirit cost of deflections. If you want to use a hard-hitting weapon, but find yourself running low on spirit due to deflect difficulty, this might be the way to go. You'll get to use a high-damage weapon, but its cost for dodging and failed deflections won't be quite so detrimental.