Well, I tried to make a mob with extra health using Attributes via summon command. But that didn't work, so I decided to use potion effects...

What I thought would happen:
Once I give them regeneration & health boost than they would have extra health. For more challenge.

What really happened:
I killed the zombie, but with the normal amount of swings with the sword, Instead of having to do like, ten swings.

How to Reproduce the bug:
put in this command:

/summon Zombie ~ ~ ~ {ActiveEffects:[{Id:21,Amplifier:(Any number),Duration:(Amount)},{Id:10,Amplifier:( Any number),Duration:(Amount)}]}