The black heart.
The corroded key will absorb 6400 damage, far more than the black heart and effective on a magic fight. For a magic fight though, use the black heart and the skeleton key as the glyph would be useless.
11 armor ~ 1 stam.
There is really no such thing as "too much" EH, it can't really be quantified. The answer to "how much EH?" is "as much as you can get".