Are invincible enemies a feature or a glitch?

[Mememento Kekri]

I just ran into an "assassin robot" with infinite health, it took about 6-7 direct dual cast giga death cross hits resulting in about 4000'ish damage but it just didnt die, is this a feature or are there just glitched invincible enemies running around?
im pretty sure ive killed "assassin robots" before so i dont know the ♥♥♥♥ is going on

[Shrinkshooter]

If it's genuinely invincible, glitch. Those are rather tanky enemies though, and depending on where you are, the same kind of enemy is buffed up. Like in the Vault you'll run into tentacle floaters, they take much more damage and drop much more gold than the exact same sprites of them you find in the Fungal Caverns.

[The Snopingasusual]

They are endgame ennemies which can spawn earlier on Fungal Cavern.
Don't try to attack them at this early state, when you will encounter them in Vaults, maybe you will have a good enough wand to kill them, they are not invicible, just very tanky.

[Alder the Dirtmonger]

Most of the robot enemies have an "armour" making lots of damage types do very little. Electricity and explosions bypass this I believe.