P3
1) No, the wall doesn't hurt, the flames are just doing 9999 damage and make you unable to move.
2) 3 divebombs and 3 divebombs.
3) If you are lucky, nothing. A BLM can stand away from the group during the second divebombs and use manawall. If you are unlucky, 8 deaths.
Bonus : Are you serious ? Do you realize than you can use a melee LB on the big snake ? Do you realize that the LB can be level 2 or 3 ? Do you realize that you can pass this phase without using the LB ?
LB3 caster : around 80%
LB2 caster : around 50%
Melee LB : kill one snake, leave a bard on the second snake and kill it when the first debuff has a few seconds left.