In the MM a zombie has the following property:
Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.
Is "damage taken"…
- the total damage since the start of the fight (which makes it impossible to pass, as the first DC is equal to 5+22+damage taken),
- total damage since first use of this property (which makes most sense to me, as a zombie becomes weaker the more damaged it is), or
- the current attack's damage (which makes a zombie a very difficult opponent for a 1st level party)?
Yesterday I ruled the third way, and four zombies wiped the floor with the party, as most attacks of my party did a mere 4–5 points of damage with no crits or radiant damage from anybody, which made it DC 9 and easy to pass. It took five turns before one almost-dead zombie failed the check.