You have the following party to work with for your ideas, so you can work from the same situation we were in. Point buy. 8th level. Against Adult White dragon in its lair.
1. Paladin: Shield Master. Defensive Fighting Style. Oath of Vengeance. Human.
2. Fighter: Battlemaster. Human. Heavy Armor Mastery. Great Weapon Master Feat. Toughness.
3. Wizard Evoker. Forest Gnome. Resilient Con feat.
4. Lore Bard. Half-elf. Warcaster feat.
5. Life Cleric. Warcaster Feat. Human.
That was our party. 8th level against an adult white dragon in its lair on an iceberg we sailed to. Think up some strategies. Not an optimal party as we found out often due to a lack of darkvision and too many melee martials.
Okay, with that party to work with against a CR 13 white dragon (Deadly threat), here are my thoughts/observations. Bear in mind that I never play clerics, so I may overlook some cleric tricks, but:
First, your evoker isn't really online yet as an evoker (that really happens at level 14), so your primary damage dealers are your melee warriors. Under those conditions, spending concentration on
Fly so they can engage is clearly your best choice. You did the right thing for the party you had. Furthermore, the key to winning this battle is going to be to concentrate all your eggs in the melee warriors' basket so they can kill the dragon.
Second, Death Ward is awesome insurance against things that have huge damage rolls, like dragons. It could be like having an extra 50 HP under the right circumstances, and allows you to not panic if a warrior goes down to only 20 HP. Your cleric can cast it twice, so it should be pre-cast on both warriors.
Third, Aid may not be huge, but it's always worth casting if you can afford it, so your cleric will pre-cast Aid III for 10 bonus HP on himself and the two warriors.
Fourth, during battle the cleric can spend his concentration on Bless to protect the warriors, and he can cast Spiritual Hammer if geography permits and then spend his actions either 1.) Healing wounded warriors, 2.) Casting Sacred Flame on the dragon for minor damage and then casting Sanctuary on whichever warrior has last attacked. Legendary Resistance is basically useless against Sanctuary, or rather it burns through Sanctuary very quickly if the dragon expends it there, rendering it more vulnerable to counterattack from the wizard and bard. I would suggest Guardian of Faith here as a good non-concentration spell, but the cleric has spent both of his 4th level slots already.
Fourth, without knowing what your lore bard's magical secrets are it's hard to say what he can do. If it were me, he'd have Conjure Animals and summon up a flock of disposable owls to clutter the battle and do damage. If he doesn't, well, I could either cast Heroism III on the cleric and warriors to protect against Frightful Presence, or I could Polymorph myself into a giant ape and chuck rocks at it. Heroism is a safer play and allows me to use Cutting Words to protect the warriors from melee attacks (expend it on rolls in the 7-11 range), and also lets me give Inspiration to boost saves/attacks. If the dragon runs out of Legendary Resistances from Sanctuary I could switch to Confusion or Hypnotic Pattern.
Fifth, the wizard will mostly be maintaining Fly IV on the two warriors, but he can spend his actions chucking Fireballs at the dragon for 28 points of damage apiece unless the dragon makes his weak Dex save or expends a Legendary Action.
Sixth, the paladin of vengeance will do what he does best and smite the dragon, hoping for critical hits to blow his big spells. And the battlemaster fighter will action surge and power attack the dragon and use superiority dice with Precise Strike to turn near misses into hits. He's got five d8s, and he's also got Bless and Bardic inspiration, so with luck he'll inflict around 75 DPR over the course of the first two rounds.
I add all of that up and it still looks pretty dicy for the PCs (I'll guesstimate 150 damage dealt to the dragon total over the first two rounds, which you said is the point where your PCs start dying--hopefully Death Ward and Sanctuary will buy some extra time), but those suggestions are the best I can do. Hopefully it's enough that they survive the hypothetical battle.