trying to keep from falling prey to too much genre-fascism; this is my own humble opinion:
look at each suffix: 'core' - hardcore. more specifically, modern hardcore, or even metalcore. and 'grind' - grindcore. both stem from hardcore punk, with influences of punk, thrash, and death metal.
grindcore: EXTREME; either sporadic or simple song structure; microsong (a song mere seconds in length); superfluous use of blast beats; heavily distorted, majorly down-tuned, thrash style; seemingly constant double picking, arpeggios, and sweeps; high and low screams and growls, with guttural, often disgusting sounding, pig, crickets, etc..
hardcore: HARD and HEAVY; more concrete song structure (intro, verse, bridge, chorus, etc.); melodic riffs involving lead and rhythm harmonies; syncopated 'chugging'; lots of breakdowns; often in drop-d, or slightly down-tuned; mid to high screaming, low growls, and clean vocals.
add some 8-bit noises, and there you go! hope that helps.
Download experimental nintendocore/metalcore from Ukraine: My Final HeadShot - ЕР "Мир в очертаньях" ( experimental nintendocore/metalcore ):