Either their families/clans/countries are enemies or it's on a more personal level where the enmity comes from both leads. 

And I'm not talking about bubbly girl vs grumpy/overbearing/traumatized boy ceo. But where they are true life and death enemies or very close to it.

I'm interested in this as well!

Hmm this is an interesting premise that’s not easy to find because a lot of the time enemies in dramas are because of opposing families, so people that wouldn’t make their children marry their rival. 

One that I think works off the top of my head is The Princess’s Man (both for the ML and the SML, because FL is the arranged marriage partner for both at different points) but I’ll try to think of more!