can someone explain what is going on here.

https://youtu.be/v5v5Mj8uR2I

I get they have health regen but i have gotten them to below half health and they instantly regen all their life. the map has no regen modifiers on it. the rare doesn't give it extra regen. this is just it's base stats with increased hp from being rare. is this enemy just immortal if you can't one shot it?

That appears to be a runic monster. Is he from an expedition (kinda looks like it as the girl is there helping you) ? There might have been a mod you added in the expedition that gave regen. Also, that monster is very cold resistant (see from yellow snowflake), so you will have tough time with him with ur build.

i don't think there was a regen mod, but there might have been and i forgot. the cold resist isn't a problem my frost bomb give 86% cold exposure and my curse gives -45%, an enemy would be hard pressed to have enough cold resist for me to have to worry about. thank you for helping.

There is a mod in expedition where monsters regenerate %life every 4 seconds.
You can always look up what mods monsters have on the expedition icon on the bottom right of the screen, left side of your spells/attacks.