My2cents
With respect to slang, unless it is part of your "cultural reality, or identity" or so popular that just about everyone uses it (for example what's up? waddup? as hello), I would avoid using it if you want to sound authentic or natural.
Of course if you want to say hardcore, then say it. But it helps if you understand what you are saying and how to use it. A lot of slang used in popular media is really a marketing gimmick anyway (like Ke$ha - originally interested in being a country music artist) and that kind of slang you just got to sell it or own it. :lol:
I agree hardcore generally just means (vaguely) extreme the authentic/real part of the meaning isn't as important here.
Have fun at your party;-)
You should also make sure your meaning of hardcore is the same as your guests or there may be some surprises!