For normal combat, it seems you target the ‘dice’ of the enemies from the front to the back. So if you had four possible attacks, you’ll target the four closest ‘dice’, no matter if they belong to the same enemy.
In abnormality combat or other special combat, you can individually choose your targets, and Ruina’s speed mechanic is in play in terms of overriding targets and such.
To redirect you need to have a higher speed than the opponent you want to redirect. Equal or lower speed will result in one-sided attacks.