Union event's aren't solo content, you can use whomever you want and it wouldn't make much difference, you'd still be clearing in the end with the help of others. With solo content, you can't rely on others to carry you and Mordred is the most important because her skills are essential. So she is top priority. Even if you don't use Mordred herself, getting Black Propaganda takes far more priority than Andromeda's ex skills, even Vivian and Cassiopeia provide better skills that they'd be worth using over Andromeda in many cases despite weaker heals, which you'll unlock both getting D'art, another top priority,