What is the best party composition for a max (Level 20) duel?

[DM who knows CR]

A party of 4 level 20 adventurers are rich enough to afford any(non home brew) magic items and have kitted themselves out as much as possible. Your goal is to gather a team that can defeat them. The Rules:
The only way to fight them (insert ridiculous plot reason) is an a 200ft by 200ft arena.
You can not exit this arena with magical or physical means until the fight is over.
The terrain inside provides plenty of cover for hiding/getting full cover with 30ft. of movement.
You will start 100ft. apart at the center of the arena.
You cannot have any spells or effects going with shorter than an hour duration(foresight or the like is fine)
You will have 2 rounds of prep where there is a barrier between you and them, and then the barrier will drop.
You cannot have any summons with you until the start of those two rounds.
What is the bast (4 players: level 20) team composition to win any duel in this format?

[Power]

I dunno, but I'd like to start with 4 College of Lore Bard 18 / Fighter 2 characters because Action Surge is very strong and the College of Lore Bards can get stupid high initiative between Jack of All Trades (add half of proficiency bonus to ability checks... like initiative) and Peerless Skill (expend a use of Bardic Inspiration to roll a Bardic Inspiration die and add it to the ability check). This is not enough for us, since we can also have the Bards give each other just 1 Bardic Inspiration die during the buff rounds and expend that on the initiative roll also (so Initiative is now +3+2d12). We add a Foresight spell (and perhaps Enhance Ability too, to counteract any disadvantage) to get advantage on the check too and add Lucky and Alert feats (so now Initiative is a bo3 d20 roll with a +8+2d12 bonus). Not being satisfied with this much, we also have all the Bards use Cutting Words on the enemy team during their initiative rolls, expending reactions appropriately (in 5E you can pretty much always use your reactions - this application is explicitly confirmed as valid by Jeremy Crawford), so they all have to subtract 1d12 from their initiative rolls too.

Where we're headed after this I don't know, but we're 9th level casters who can take spells from every spell list and at least initiative should be a knockout here. The Bards should also all have Counterspell, because they get Jack of All Trades and Bardic Inspiration on that too (although a 9th level Counterspell just auto-counters everything, which can lead to counterspelling wars). I guess the Bards should also all be pre-buffed with True Seeing and have both proficiency and expertise in Perception (so +12+1d12), and maybe get some more buffs on that too.

[DM who knows CR]

That is strong, but can be defeated pretty easily by an opponent striker (striker meaning they focus on going first and dealing a lot of round 1 damage). For example, A Haregon with bladesinger 6 / echo knight 3 / Fiend warlock 4 / Assassin 3 / Sorc 4 with The Alert feat, A bard on their team, An artificer on their team, Scorpion armor, Superior technique fighting style, lucky, and enhance ability would have 3 roll advantage, (+1d12(bardic insp)+1d6(sup tec)+dex+6(Haregon)+5(alert)+5(scorpion armor)+5(flash of genius)) for a total of(+1d12+1d6+16+dex(probs 2/3)), AND they can nullify the effects of cutting words by wearing ear plugs (because otherwise cutting words would be very strong against this). +8+2d12 (we will ignore dex) vs +21+1d12+1d6
average of +21 vs average of +31
with tripple advantage the odds of any of the bards beating this guy are about nill In addition this guy can pump out enough damage in round one to potentially end the fight then and there. (High level scorching ray (BA-quickened)+Use eldrich blast as your cantrip+ use a bow for regular attacks (action surge) (everything crits(assassian rouge))) Combine this with an item like the Teeth of Dahlver-Nar 4th implanted effect, and you can almost guarantee that those bards are dead, but there are counters to this, and having at least one bard is very important. In addition you didn't specify any magic items and those could help you greatly.

[Power]

It's silly but you could use the Telepathic feat (and pre-buff with a Tongues spell to be absolutely safe) and a Contingency to Dimension Door the Bards forward the moment initiative is about to be rolled. That would ensure they cannot plug their ears against it. If the party also uses Scorpion Armor (they're multiclassed Fighters, so heck we could have them start with 2 levels of Fighter for the free proficiency) and cast Gift of Alacrity on themselves (another 1d8 bonus), it gets even higher. I guess we could have the Bards be Haregons too, for the extra +3. If we're being maximally silly every Bard should also have their own Sword of Kos for an extra 1d10 to initiative with the Guardian minor magical property for an extra +2. I guess we could also have the party freeform Wish ahead of time for a massive bonus to our next initiative roll?

We also don't have a clear idea how high everyone's ability scores are, since there are ways to get multiple ability scores as high as 30 and make a multiclassed build to keep adding ability scores to initiative.