Im currently 12BR with 4 unions and 12member max, I have tried putting a mixture of people in each union but learnt early in the game the party members say stuff like "what are you doing!" cause skills dont match so people are left to do nothing but melee swing.

So I just tried doing the logical thing, I made 3 unions, 1 full of tanks, shields high def, high HP stacked HP and resist trinkets in parties slots(PC version with tweak to equip party members).
2nd group was a healer/debuffer/ranged group with 2 healers, a specialist(hexer poisoner) and a marksman.
3rd was a melee DD unit that was stacked for slash and the formation to enhance slashing.

So I go into battle, tank union has specials switched off so they can class change to monks:

quote tlr wiki

HP + 5 / AP Init + 4 / Defender Item Arts > Mystic Arts (PC) Item Arts highest, Only base attacks used no Combat Arts/Mystic Arts and not higher than rank one, STR > INT (NOTE: Expert Fighter Bonus lost)

I tell the healer union to stay back and heal as I dont want them deadlocked.

3rd union I was hoping to have build AP or intercept a union trying to flank(i dont even know how to do that!)

Anyways the problem is the healer group continuously gets flanked and deadlocked which forces my DD union to flank that union but when there is 3 or more enemy unions the healer union dies very easily, the whole thing is just so messy it obviously isnt working.

So is there even a valid strategy for this game or is the combat system broken or maybe just misunderstood by everyone??

i always make all my unions all-rounders, the game gets really tedious if you rely on one union to do a certain thing, especailly healers when they get flanked

everyone one of my unions has good mystic,combat and healing arts and ive never had trouble beating the game,

its just the bloody fallen who i had trouble with but tbf i wasnt high enough BR