While I may not have completed some of these stages I do know what seems to be their weaknesses with pokemon that are catchable before beating mega gengar (I hate you mega gengar) as it seems to be putting me pretty close to beating the stages.
Raichu use mega kenga, the rows of pichu/kachu are impossibly to remove easily any other way and by using mega kenga not only are you dealing a decent amount of damage, but if you clear the top 3 rows you have another 3 rows of pokemon to match etc. I tested this once, on a theory and got S rank on my first try.
Blaziken -arodactyl and keldeo arodactyl slowly removes the clouds (which are annoying AF) and keldeo removes the constant double row of unbreakable blocks getting spammed every 3 seconds.
Xerneas -remove the top frozen/barriered pokemon as quickly as possible, those rows of trapped pokemon basically lock you into a stupidly small area with in 5 moves if you don't remove them as your first moves.
Raichu use mega kenga, the rows of pichu/kachu are impossibly to remove easily any other way and by using mega kenga not only are you dealing a decent amount of damage, but if you clear the top 3 rows you have another 3 rows of pokemon to match etc. I tested this once, on a theory and got S rank on my first try.
Blaziken -arodactyl and keldeo arodactyl slowly removes the clouds (which are annoying AF) and keldeo removes the constant double row of unbreakable blocks getting spammed every 3 seconds.
Xerneas -remove the top frozen/barriered pokemon as quickly as possible, those rows of trapped pokemon basically lock you into a stupidly small area with in 5 moves if you don't remove them as your first moves.