Summon Board Passive selections:
Odin choose 3 / 6 / 7
Bahamut choose 3 / 4 / 6
Usuals:
Ifrit, Shiva, Ramuh, Brothers 4 / 5 / 7
Leviathan, Pandemonium, Diabolos, Alexander 4 / 5 / 6
For more details and help with summons, check Summon Hub!
C Spheres: Paladin Cecil or his RF sphere are best for both of these slots. 2 RF are fine as they will stack. If no Paladin Cecil available, Alphinaud or Maria is an average alternative.
D Sphere: Any ATK sphere will do the trick here, just avoid Wakka unless you can apply 5 buffs on her.
Rosa with her BT effect becomes an effective attacker thanks to Holy Blade followup. Her rework also gives her the important buffs she needs at Quest Start to really hit the ground running.
If you're racing the enemy FR gauge, spam that S2 like there's not tomorrow. For FR time, have her BT effect prior to the FR attack so you can have those juicy follow ups after her turns. If no one is using their BT phase, LD, EX are her hardest hitting normal buttons and FR echo when you need to (both early, one early & one later or both at the end).
Divine Blast (S1) is a filler skill or you lack BT and need some emergency healing while EX isn't up.
Pray (S2) is her FR gauge charging skill and it maintains her buffs while charging making it very convenient.
Luminous Arrow (EX) 2 skills to recharge roughly, but may take 3 skills in weird speed/Aim things.
It offers a free skill use, however Divine Blast is weaker than Aim in Force Time so you'd use Aim in Burst Phase. Pray may be useful if you're racing the enemy FR gauge but in Force Time you're probably better off dealing tons of damage with an LD or FR.
Blessed Archery (LD) refreshes her valuable LD buff that provides the party with more support and unlocks her unique BRV attack - Aim.
Aim hits very hard however it's mainly single target focused and is still weaker than her EX and LD in a 2+ target scenario. It's mainly used if you choose to BT phase with Rosa.
Holy Cross (BT) unleashes her ultimate power, Cecil.
He follows up Rosa's turns with a 3 Full HP attack, yes, turn. So if you really wanted to use Pray, he'd follow up then too.
Black White Barrage (FR) is a nice heal and generic FR time many users can enjoy.
Active member is full HP (or overhealed): +45%
Party has 10+ buffs: +30%
Consider using Hope or Sazh calls to add more buffs to the party and max her LD buff stacks. Other calls also work, these are just examples that also are useful in their own right if allies lack buffs.
Some Crystal Passives provide buffs which can help raise her LD stacks as well
She is an attacker now and can work with other strong attacking-supports or she can support attackers that need the help while pumping out damage with them.
Chain Bonus (this passive belongs in a trash can on fire at the moment of the destruction of the universe)
HP Attack++ Up (never used)
Aim Up (rarely ever used)
Blessed Archery Focused Attack (useless in AoE fights, but important in single target fights)
Break Bonus Up (marginal effect, boosting Break Bonus by 20% when she inflicts a break)