What are the best buff spells for a 10th level party to use on a fighter for a 1v1 arena vs a dragon?

Question

A party consisting of a 10th level fighter, wizard, druid, and cleric are in a situation where the fighter will be planeshifted into a planar arena to fight a dragon. They don't know the nature of the dragon. Nothing stops the party from casting spells on the fighter beforehand, and they have access to a mages' guild that will sell them magical scrolls and potions, within reason (no Wish casts or w/e).

Assuming they are willing to expend any amount of resources to get every applicable buff onto this fighter, what buffs should they be aiming to get onto them? Once the fight starts, they can't aid the fighter further - it's strictly a 1v1. The fighter is a generalist who uses a greatsword in melee and a longbow at range, switching to sword-and-board vs swarms of enemies, with fairly average stats - 18 str, 16 dex, 16 con, 12 wis, 12 int, 12 cha. As magic items, the fighter has +2 weaponry and a suit of +2 mithril platemail.

The allowed material is any published 1st-party 5e content (this includes wildermount etc). The type of dragon is not known because the fight is in Tiamat's Lair, in Avernus (where there are many dragons). Divination could theoretically predict the kind of dragon being used to fight but that's a very fiat thing and presumably the devils and dragons there could use some sort of bafflement to mess with it. The only known thing is that the organizers of the fight underestimate the party, meaning they are unlikely to pull out any big guns.

Answer 1

Cost vs. Benefit

So there is a pretty obvious answer that is, all of them. Sans duplicated effects, buffs can really supercharge characters especially at the top end. The 9th level Foresight is an incredible buff in its own right and when combined with other effects it can make characters punch far above their weight class. But if you want it on the cheap, then it gets more interesting as to what is most effective. Without the guild's help, it's gonna be pretty difficult to win this fight unless the fighter is going up against a Young dragon. So I'm going to subdivide the lists into how much the party is willing to splurge.

Just the Party

If we're using party buffs alone, then the list is pretty hefty simply because you need all the help you can get.

Cleric

• Shield of Faith
• Aid
• Freedom of Movement
• Death Ward

Druid

• Longstrider
• Darkvision
• Stoneskin

Wizard

• Gift of Alacrity (paywalled)
• Fortune's Favor (paywalled)
• Haste

Limited Help

Assuming modest funds, the party would likely be able to afford a couple rare potions and maybe a sixth or seventh level buff (assuming the cost of applying the buff is the same as a scroll).

Potions

• Potion of Giant's Strength (Fire)
• Potion of Heroism
• Potion of Invulnerability

Cost efficient buffs

• Hero's Feast

Supercharges

If we're really saying money is of no consequence then as mentioned before we can get pretty wild. I'd dare say the fighter could take on a full adult red with just the buffs listed below.

Potions

• Potion of Giant Size (paywalled)
• Potion of Giant's Strength (Storm)
• Potion of Flying
• Potion of Possibility
• *Potion of Speed (*Replaces the mage's Haste in case of a spellcaster dragon)

Cost efficient super-buffs

• Foresight

Cost Conclusions

If you're gonna fight a dragon, it's always best to be prepared. And some buffs give you that prep for incredibly cheap. With just the party you can provide an array of defensive options with a few offensive upgrades. It might not be enough to go toe-to-toe with an adult dragon (even the lower CR flavors) but it'll put you in the ball park. And with some limited help you can expect a fighter to punch seriously above their weight. But once you input unlimited money hacks, you can truly get a Fighter to some incredible heights.

Priority Buffs

All of the party buffs should be key since the 1 minute time-limit buffs can effectively be applied the moment before the fight begins. Even the longer duration buffs are worthwhile since they limit shenanigans (like getting restrained or a bad round). Immediately after that would be Flight to limit the dragon's one key advantage. The potion of invulnerability would follow as it limits the dragon's damage incredibly. Finally, if you can only afford one, Foresight or a Potion of Giant's Size is a hard debate but I would choose Foresight because 24 AC will be tough to crack (Haste + Shield of Faith) so giving disadvantage while you maintain advantage helps every attack vs the higher damage of the great sword when you get close.

Answer 2

Energy resistance and fast flight are your friends

Guiding Olive's excellent answer already covers all the buffs that make sense comprehensively.

A likely strategy for the dragon (who will have a fly speed of 80 as an adult) is to avoid melee combat with your heavily armored fighter, and focus on flying above using its breath weapon. If that happens, the fight will turn into a damage race of your longbow vs its breath recharge unless you can outfly it.

For this reason (and because you do not know the energy type the dragon will be using), the potion of invulnerability is probably the most important item to purchase.

Even with longstrider and fly your 70 feet fly speed would not match this. If you can also get a potion of flying and combine that with your wizards haste prioritize it: with longstrider it will give an effective fly speed of 70 feet and an extra action you can use to dash, for 140 feet, outdistancing the dragon's fly speed by a fair margin, so you can get up close and personal and make your AC matter.

By your description, the guild only provides access to potions and scrolls. If they also have other consumables, a couple of arrows of dragon slaying can help tilt the ranged damage race in your favor, too.