You need to experiment and see what combinations work for you, for a given party composition and enemy type. Look for skills that make the enemy more vulnerable to other attacks, particularly Mark abilities. Something that may not be obvious is that for characters that have more than four skills unlocked you can switch between them when out of combat.
Except for certain bosses that get several action per turn, the only way to act twice in a row is to get the last action on one turn and then the first action on the next. If an enemy is stunned, the first of those actions would be skipped and the stun cleared, then they get to take the next action normally. Stunning enemies doesn’t affect turn order or gives extra moves.