Trance is more plot-related than battle-related, and you'll discover more of this towards the end of disc 3. I think it's good they've made it this way. In FFVII you could save your limit breaks for bosses. In FFVIII the system was completely broken and could be abused for the entire game.

With Trance you need to play it tactically. If you know a boss battle is coming up, and you should as you're using a guide, you can build up your Trance bar in random battles, leaving just enough to make sure it activates when the boss attacks. You can Flee from random battles if you don't need any more damage.

As you've discovered, High Tide doubles the amount by which the Trance bar is increased. Having a high Spirit stat increases the amount by which the bar is filled, as well as determining how long the character will remain in Trance.

Saying that, Trance isn't really needed, considering the highest HP you'll encounter is 65,535. The character's normal attacks and abilities deal more than enough damage to make do with.

As for Zidane's Dyne skills, here's what they do.

Free Energy: Attack Power * 1.5 single enemy
Tidal Flame: Attack Power * 1.5 all enemies
Scoop Art: Attack Power * 2 single enemy
Shift Break: Attack Power * 2 all enemies
Stella Circle 5: Attack Power * 2.5 single enemy
Meo Twister: Attack Power * 2.5 all enemies
Solution 9: Attack Power * 3 single enemy
Grand Lethal: Attack Power * 4 all enemies

All attacks are physical and non-elemental.