Our group can beat Bahamut Coil T1 ~ T4 with no problem, but cannot beat Turn 5 at the moment. I'm wondering what should I use my myth for?

Here's my current gear/stats:

race: Hyur - Midlander

weapons: Artemis Bow Zenith

head: Bard's Chapeau
body: Bard's Shirt
hands: Ballad Gauntlets
waist: Ballad Sash
legs: Bard's Tight
feet: Ballad boots

neck: Darklight Choker of Aiming
ears: Darklight Earrings of Aiming
wrist: Darklight Bracelet of Aiming
ring left: Vortex Ring of Aiming
ring right: Hero's Ring of Aiming

stats:
Dex: 474
Accuracy: 479
Critical: 491
Determination: 256
Skill Speed: 396
Vit: 370

I'm thinking my priority for AF2/myth should be:
hands > feet > waist > earring > wrist
(i'll try to get my neck piece from Ultima)

I also have 2 other questions:

1. How much accuracy is needed for Turn 1~Turn 4, and Turn 5 (for both melee, ranged melee, blm)

2. What's the conversion rate for critical vs DEX and Determination vs DEX?

Thank you