2. Overdrive meter is symbolized by the little yellow dot. Each time an enemy attack, he gains a dot. When his bar is full, he will release a powerful attack next turn.
Gauge is the rage bar under the boss hp. When its filled, and as long as you didnt fully deplete it, it will boost overdrive attack a lot. You want to avoid it at all cost.
Once the rage bar is empty, the boss is stunned. While in this state, he wont gain overdrive charge with his attack, and wont be able to use it either.
1. Sieg's morale boost will allow your team's attack to have more impact on rage bar, so you can deplete it faster.
Sorry to disappoint you but you can only get Thor from premium gacha. The weapon you get from divine souls doesnt release any kamihime.