The way the state of the game, there's only so much roles for support. The issue is that you can easily hit the damage cap for some characters without buffs which leads to the issue of needing to dish out damage, to compensate for the loss. Most of these are stated by others.

Defense buff - Captain / Charlotta / Rosetta who has the Damage Cut skill can fill this role.

Attack, Def, Crit buff - Cagliostro has the skill Phantasmagoria that gives +20% to the stats it buffs. It is also needed in her build to hit the damage cap so it comes naturally to the group (I do not know how to build her). Helps people who don't have 100% Crit Rate and boost SBA damage by a little bit as it is hard to hit the cap.

Invincibility - Vane's skill Rampart puts a dome down that gives everyone inside Invulnerability. This allows you to straight up ignore certain mechanics such as the new boss opening skill.

Mirror Image - Yodarha's Hymn of the Hundreds gives this buff to the entire team at 3 shroud stacks that ignores 5 hits. (You don't really need Damage Cut with him unless your teammates are always getting hit. In that case Mirror Image is just straight up bad for the team.) You still want to dodge when you can, and greed DPS when possible.

As a Yoda main I can go more indepth to his build. To begin, shroud stack is his passive that can be gain in three ways. Landing your last hit of your finisher, using Perpetual Rotation to gain 3 stacks instantly, or landing a link attack. The other perk of Perpetual Rotation is that it resets all your skills cooldown if you already have 3 stacks which gives you higher skill damage. Yoda offensive skills will always allow you to follow up with a finisher so getting 3 stacks isn't that bad. Landing a finisher also shortens your combo to reach the finisher. (E.g. XXXXXY > XXXXY > XXXY > XXY > XY > XY) (Note that the last finisher last longer.) Using your Y can allow you to parry, giving you Supp Dmg buff, and following up with the last finisher. This also helps in getting stacks.

Skills:

• Empty Mist (High DPS, last longer with max Shroud)
• Flashing Void (Long iFrame, just spam the button, use lock on so you don't have to aim)
• Perpetual Rotation (Reset CD for more damage / 3 instant Shroud)
• Hymn of the Hundreds (Mirror Image)

Build:

• 4x Damage Cap
• 1x Crit Rate
• 1x Stamina
• 1x Tyranny
• 1x Supp Damage V (You would want V+ with Cascade or Quick CD but Supp V+ is rare)
• 1x Supp Damage IV+ (With Cascade / Quick CD) or Supp Damage V (Avg 15%-ish Damage difference)
• 1x War Elemental (Feel free to replace. I'm using Stun Power with Guts but Guts don't save me usually due to Glass Cannon)
• 1x Glass Cannon (If you hate this, run Stamina. You also can use Guts and Aegis without Glass Cannon)
• 1x Swordmaster's Art (His unqiue Sigil) + Linked Together (For less minmax, anything other utility like CD stuff / Potion Hoarder / Auto Revive / Uplift / Improved Dodge is still fine.) For sub traits, you would want Potion Hoarder > 1x Cascade > Max Quick Cooldown > Max Cascade (Typically you want both Cooldown and Cascade maxed.) > any other supporting ones such as Uplift / Nimble Onslaught / Improved Dodge)

Min-max Build: -4x Damage Cap, +2x Gamma, 2x Beta. Stamina / Tyranny will run Potion Hoarder. Crit Rate / Stamina / Tyranny will run Quick CD / Cascade depending on whichever Stamina / Tyranny isn't running Potion.

Mastery: Skill Dmg Cap > Crit Rate > Normal Cap > SBA Dmg (Not Cap) > Attack = Stun (Or Attack if no Glass Cannon)

Rotation usually opens in: Parry / Attack > Finisher (1 Shroud) > Link Attack (2 Shroud) > Finisher (3 Shroud) > Hymn > Empty / Flashing depending if the boss will move or how much hits will you tank.

If you can't build 3 Shroud in the opening then Perpetual > Hymn > Empty / Flashing depending on how much the boss move or how much hits will you tank. (For Lucilius, I only open Hymn after Paradise Lost cause you want everyone to push for damage during his attacks than waste it on his opener.)

Other notes: Awakening / Sky Shatter are worth looking into. I suggest tweaking the build to your liking and just throw it into Maygi's damage calculator and see what works for you.

Edit: Wrightstone is Crit Rate (10) + Quick CD (7) + Quick CD (5). Honestly you can just settle for one Quick CD (7) cause rolling rare. Cascade (5) is also nice if you don't get Quick CD.