Optimizing party make-up in FFRK

The other answers seem very good but I believe they left a very important thing out: survivability.

Those strats are all good and well to do damage but you still need to survive. This is most important vs those level 120 bosses.

You need to figure out what sorts of damage the bosses have.

1) If they have heavy hitting magic damage have your healer bring Shellga and Protectga if it's physical damage. You only need these abilities lvl 1 or 2 since 2 casts are usually sufficient to defeat a boss.

2) I maxed my magic break, power break, armor break, and mental break. Typically, I have 2 physical attackers with these 4 abilities equipped. I generally will just alternate which one I use each round.

This significantly negates damage and also makes hitting the bosses weaknesses much stronger. It's pretty normal for me to use a Level 3 spell and hit for over 9k against their weaknesses with resistances dropped. Also, even though bosses favor magic or physical, they will still often have some of the opposite damage so using the power/magic break will help mitigate that. Even though you have shellga or protectga already to mitigate their primary damage, using the associated power/magic break reduces that damage even further. There were several level 120 bosses that would hit either all or part of my party for over 2.5k damage with their primary attacks. With the protectga/shellga and power/magic break it usually knocks that damage down to somewhere between 500-800 which is far more managable.

Typically, my party formation is something like:

2 Physical Attack w/breaks 1 Healer 1 Black Mage 1 Flex Pick

My Black magic Users typically do the most damage from their abilities. They have issues clearing levels but often the hardest part is the bosses and so I want to clear them as fast as possible. I also have a lvl 3 Quake to help clear stages on the way to bosses if I need to conserve hp/ability uses for other characters.

My Flex pick is usually my key to beating specific bosses though. The main focus of this pick is exploiting a bosses weakness and occasionally if I need a little extra support to survive particularly vicious bosses.

Bosses usually fall into 3 and almost 4 categories for me.

1) If a boss has an elemental weakness, I'll usually add a spellblade into the party to exploit it on top of my existing black mage. They are great for clearing previous stages to get to bosses since they have high physical attack as well. Occasionally I will add a second black mage if I have the available spells to take advantage of their weaknesses. If I use a second black mage, I'll often have quake on one of them just to help clear early stages since all the clearing needs done by 2 physical attackers if you don't.

2) Boss has a vulnerability to a debuff. This is usually very similar to the above category. Instead of a spellblade though, I'll take another fighter who can exploit the vulnerability. For instance, if they are weak to slow, I'll take a fighter who can use Tempo Flurry. I choose whatever fighter can use another useful ability on top of it such as Dismissal. Sometimes I'll bring 4 physical attackers in this case (also a great party to use the retaliate strat others describe above, I still prefer having some of the armor/power breaks/magic breaks in this situation). Similar situation for blind. Sometimes a fighter won't be able to inflict this vulnerability such as Stop. At this point I can just have my black mage to take it which still leaves me with a Flex pick to make any party comp I want.

3) Some bosses will just tear your party up even with all the damage mitigation I describe above. In this case you might need a Flex pick that will give you a little bit of healing utility as well as able to output damage. Summoners who use black magic can often do this. Using Kirin can help out your primary healer when an enemy has a decent amount of AOE damage that adds up. Other times you can add another physical attacker who also has access to level 3 healing for Curaga when you need it.

4) Bosses with no weaknesses and a little bit of everything. These can be a little frustrating since there is nothing to exploit for them. Your best bet is just using this last flex pick to create a specific party comp. At this point your black mage becomes a second flex pick as well since they won't be required. Build whatever comp you want and have at it. Great for using those retaliate comps described in other answers.

EDIT:

Focus on getting a full party with variety to 80 before worrying about specific synergies.

Here are a few strategies I have used for higher level difficulty dungeons. Target conditions and boss weaknesses should be considered when completing any dungeon and should be accounted for when determining whether or how to use any of the strategies below.

Advantaliate:
Summary:

Stack Boost and Advance SB from Luneth RW on character with retaliate to allow all party members to hit like a truck. Best used in a melee heavy party against bosses that do not have super high defense.

Setup:

• Equip your strongest weapon on a character that can equip retaliate. Cloud/Sephiroth are generally accepted to be the best retaliators, but Cyan/Tyro can use it as well.
• Include a character that has at least Support 1 and equip Boost on that character
• When selecting your roaming warrior, select a Luneth RW with the Advance SB
• Equip as many party members as possible with either honed Double Cut or Double Hit record materia (Josef RM2)

Use:

1. Retaliator cast Retaliate, Boost cast on retaliator, apply any other buffs (e.g. protectga, shellga) or debuffs (e.g. armor/power/magic break[down])
2. Retaliator cast Advance (from RW), rest of party targets retaliator and uses double cut/attack. Healer can heal if necessary
3. Retaliator cast retaliate, rest double cut/attack retaliator or heal/buff/debuff
4. Retaliator attack/use 2nd ability, rest double cut/attack retaliator or heal
5. Go to 3.

Tauntaliate:
Summary: Use either Sentinel SB from Paladin Cecil RW or Draw Fire ability on character with retaliate to force enemies to attack retaliator. Use Boost on retaliator for damage increase. Best used in a melee heavy party against bosses that use very few or no magic attacks.

Setup:

• Equip your strongest weapon on a character that can equip retaliate. Cloud/Sephiroth are generally accepted to be the best retaliators, but Cyan/Tyro can use it as well.
• Include a character that has at least Support 1 and equip Boost on that character
• Two methods of using tauntaliate - select Paladin Cecil RW with Sentinel SB or use Tyro as retaliator and bring Draw Fire as second ability.
• Equip as many party members as possible with either honed Double Cut or Double Hit record materia (Josef RM2)

Use:

• Retaliator cast Retaliate, cast Boost on retaliator, apply any other buffs (e.g. protectga, shellga) or debuffs (e.g. armor/power/magic break[down])
• Retaliator cast Sentinel (from RW) or if Tyro with Draw Fire, cast Draw Fire, rest of party targets retaliator and uses double cut/attack. Healer can heal if necessary
• Retaliator cast retaliate, rest double cut/attack retaliator or heal/buff/debuff
• Retaliator attack/use 2nd ability (if Tyro with Draw Fire, use Draw Fire), rest double cut/attack retaliator or heal
• Go to 3.

Carbuncle/Kirin:
Summary: Use Carbuncle to reflect single target magic attacks back at the enemy. Kirin used for healing/negative status cleansing. Best used on enemies that always or almost always use single target magic attacks. AoE magic does not get reflected!

Setup:

• Bring a Summoner with access to at least summoning 4 and equip Carbuncle and Kirin on them

Use:

1. Cast any single target magic based buffs (before Carbuncle), Summoner cast Carbuncle, rest can attack/debuff/whatever
2. Summoner cast Kirin, rest can attack/debuff/whatever
3. Recast Carbuncle/Kirin if necessary, otherwise all party members attack/debuff/whatever

Runic Spam:
Summary: Celes' Soul Sabre SB (Runic) absorbs any single target magic attack cast against any party member and restores one ability use for both abilities equipped on the character using the Runic SB. Use Runic on a Black Mage/Summoner to refill ability uses on high damage spells as you continue to use them on the enemy. Best used on enemies that always or almost always use single target magic attacks. AoE magic cast on your party by the enemy will damage everyone but the Runic user (it will restore one ability use on the Runic user).

Setup:

• Equip a Black Mage/Summoner with spells/summons that will do high damage (e.g. quake, Valefor, -ga/-ja spell that is elemental weakness of boss)
• When selecting your roaming warrior, choose a Celes RW that has the Runic SB

Use:

1. Black Mage/Summoner cast Runic (from RW), rest cast any buffs/debuffs/attack
2. Black Mage/Summoner cast high damage ability, rest attack/heal/whatever
3. Black Mage/Summoner recast Runic if necessary otherwise cast high damage ability, rest attack/heal/buff/debuff/whatever

There are other strategies you can use depending on the event dungeon, but the retaliate based ones are pretty powerful when you have a 4* or better synergy weapon. Hope this helps.

Note that what you can build for an optimal team will be strongly dependent on your strongest available equipment and Soul Breaks. Here is general strategy which you can optimize per your own team's strengths.

Short Version - My build has:

• Cloud - melee fighter, debuff or debuff/damage mix
• Wakka - melee fighter, heavy debuff
• Vivi - offensive magic
• Yuna - healing and offensive magic
• Garnet - healing and buff magic

Long Version - At this level, many of the strongest bosses have abilities that can one-shot a strong character, deal significant group damage, or both. Consequently, defensive skills are more important then that have been for previous battles. Additionally, the boss magic attacks generally pack more punch than physical attacks. Out of your 10 ability slots, 3 should be Power Breakdown, Magic Breakdown, and Shellga (Note that there are not many characters who can use the 4-star Breakdown skills. I have been using Wakka with a 5* katana in the front row.)

Against a boss with strong physical attacks, you may want Protectga as well as (or in place of) Shellga.

Unfortunately, this leaves you with relatively few offensive slots. You need to make sure the ones you have stretch as far as possible. To make up for this, use a strong melee fighter (Ex: Cloud) with Mental Break.

Vivi is the mage of choice due to Focus Magic soul break. When focus Magic and Mental Break are both active, 4-star spells should be doing 9999 damage per round. Equipping a proper Record Materia (such as Rod Master) makes this even more likely.

Obviously you also need a healing ability such as Curaga or Curaja. Which one you pick depends on how honed they are. Since this team build reduces damage spikes, a 12-use Curaga will be more useful than a 6-use Curaja. I generally don't bring Raise. Having a character die will generally tank your ranking by so much that you can't get a Master rank.

If you are using two white mage characters, offensive holy magic such as Diara or Diaga, especially with Prayer of the Cetra record materia, will often provide a significant amount of damage over the course of the fight.

The first bonus fight in many events is a boss marathon. You may need to burn 1 or 2 Mythril to get Master rank on those with this team. That's OK.

Finally, you will notice that some bosses (such as the Emperor in FF2) are listed as having resistance to debuff. Don't worry about it - the debuffs are still valuable enough to have even in these fights.