Once in a while I will practice technique and when I do I want it to be as effective as possible.

One favourite is doing harmonics with left hand only (I am right-handed) especially the pinky which is the weakest link in the chain so I would pull the string with the pinky and create the harmonics with for example ring finger while the other fingers are pressing down strings. Headache-inducing at times but effective

What's your take?

my idea of practice is playing unplugged without a pick. chord progressions and "plucked sweeps" stuff like that.
but then again you can't really practice any "metal" techniques that way.
one thing I do which is kinda metal is to practice playing little licks with my picking hand, great if you want to better your tapping skills.
another good picking hand shtick is to follow your fretted note with an harmonic produced by your picking hand an octave higher.
(place thumb above desired fret and use your ring finger to pluck-this is actually a bass thing)
kinda useless but I'm not much of a guitar virtuous.