How do i add the Reverse Mode option for fe 6,7 and 8?

Questions
2

In fe8 hacks with skillsys, you can give enemies the vantage+ skill. You’ll have to assign the skill for each enemy, which could get tedious.

If you’re fine with breaking vantage for players, it’d be easiest to just replace this function.

void BattleGetBattleUnitOrder(struct BattleUnit** outAttacker, struct BattleUnit** outDefender) {
    *outAttacker = &gBattleActor;
    *outDefender = &gBattleTarget;
}

This guide can help you to easily compile the c code:

void BattleGetBattleUnitOrder(struct BattleUnit** outAttacker, struct BattleUnit** outDefender) {
    *outAttacker = &gBattleActor;
    *outDefender = &gBattleTarget;
    if (gBattleTarget.unit.index & 0x80) // enemy
    { 
        *outAttacker = &gBattleTarget;
        *outDefender = &gBattleActor;
    } 
}

Something like this might work.