I can't speak about every boss because i've never fought all of them,but to half-say that Pangoro's Team with Mementos is the only way to beat hard boss is very wrong,there are a lot of strats to win,find one(or many)that will work against each boss,that's the key,of course there are useful videos guides on Youtube and writed ones on forum,but it doesn't mean that we have to follow them at 100%,try the strats that you feel and see what works or not against a specific boss.
I don't have Pangoro,i don't use Power Trip and i win a lot of boss and sometimes all of them(20),if there are no bad surprises on my side(you know what i mean)in these battles,so the others options are already there,it's for you to try.