I just force then to faint, then revive them with a Revival Herb. Keep going until you know they're on 0 happiness. It's easiest to do this after right after you've traded them so that they are at their base happiness. Sal's base is 35, so he only needs to faint 3 times to be at 0. If he's too powerful to have wild Pokemon / trainers KO him, give him a Toxic Orb and just wait it out
I've found 3 Japanese users on Twitter with that TSV, they are
@ecreacrea,
@p_stoneedge and
@AyuKashigen (the two first ones seem fairly active). There's a
guide on reddit for how to talk to them incase you don't speak Japanese / don't like using Google Translate.. I've used the Google Document that's linked to hatch a couple thing's myself, it's super easy and the people are really nice
e: used 'two' instead of 'too.' time for bed...