Use Moonblades
The basic idea here is to use infinite damage moonblades to do infinite damage per hit. However, since our damage is limited by the hp of our opponents, this does not convert to infinite damage.
Step one-- change form
A tankard doesn't get multiattack and we can't easily give it levels in fighter. Ergo we must first change its form (probably via True Polymorph back into a regular tankard and then True Polymorph again into a CR 9 tiny monster and then again into the final creature, the second overriding the 'less potent' polymorph, and then itself being dispelled via Dispel Magic, but anything that replaces statistics can work) so that it gains multiattack. The best we can do appears to be three attacks via the Gladiator-- while the quadrone gets more attacks and they are with weapons they are only with bows while moonblades are longswords.
Moonblades only bond with elves and half-elves, so normally we'd have to change into an elf, attune the moonblade, and then change into our final form. However, since gladiators can be elven, we can just go straight to being an elven gladiator.
Form changing is allowed by your question, of course, but if you don't like it, just subtract two attacks per round from the final result and use the Fates card method in the 'improvements' section at the end-- the rest of the work will be the same.
Step two-- buff
We want buffs. Our main character will concentrate on Haste for us, cast Wish to duplicate the effects of True Seeing for us without the 25 gp component a round before the fight, and give us a Sanctuary effect on the first round. We will also be subject to Fire Shield (both versions), Hallow with Extradimensional Interference, Foresight, Freedom of Movement, Mind Blank, and Longstrider.
Additionally, we will be mounted upon a Broom of Flying, wearing attuned Claws of the Umber Hulk, assuming 'can't manipulate objects' is taken to mean 'can't Use an Object action' and not 'can't wield weapons, wear armor, consume rations, or otherwise do anything involving objects', and wearing an attuned Belt of Storm Giant Strength. Our Gladiator also is attuned to a dancing moonblade with infinite runes.
Our moonblade can store spells in it but there's nothing great 5th level or lower to put in it, I don't think, since we already have haste. Counterspell seems like a good choice.
Step three-- combat
On the first round we are well-protected against hostile immobilizing effects by our myriad defenses, especially Sanctuary. Once it is our turn we fly, burrow, or run over to the nearest enemy and Multiattack it with our dancing moonblade. We have advantage on the attack because Foresight, and with our belt and moonblade we have a total to-hit bonus of +15. We hit on a 5+, which is a 96% chance of success. We kill anything we hit with our infinite damage, dealing its hp in effective damage. We've now broken Sanctuary so enemies no longer have to make a DC 24 Wisdom save to do anything mean to us. We then continue moving to the next target, hitting it with a 96% chance of success as well. We do this again for our final attack of our multiattack, then for our Haste attack, then loose the weapon to attack with a bonus action via the dancing property, again with a 96% chance of success, using our free object interaction to pick it up again when it's done, positioning ourselves so that an enemy will, it its insane hubris, move right past us taking the shortest route towards the main character, thus provoking an attack of opportunity.
On our next turns, we continue to attempt to kill up to 6 enemies per round with a 96% chance of success each. We have advantage on any saves they might make us make, and protection (including retaliatory damage) if they give up on the whole infinite hubris thing and attack us. We can deal with flying, burrowing, invisible, ethereal, teleporting, and climbing creatures. We deal infinite slashing, fire, cold, acid, lightning, and thunder damage, so unless a creature is immune to all of those (e.g. because it is subject to an ongoing Invulnerability spell) it will take infinite damage on a hit and thus die.
We may have to adjust our tactics if the enemies prove especially problematic. For example, we can only go 40 feet underground per round so if there are only enemies underground and they are far enough apart we may not be able to get 6 in a round. We are immune to most control effects, but certain readied actions, like a readied wall of force or antimagic field could prove disastrous for us. If creatures ready such spells, we need to avoid them. Similarly, if a creature readies an action to grab our dancing weapon, we may need to kill that target normally to avoid risking a <4% chance we lose our weapon after it misses and their attempt to grab it is successful.
Assuming the creatures helpfully march forwards and let us kill them, though, we do an average of 96%*6*their average hp in damage per round.
Improvements
Swift Quiver is faster than a dancing moonblade, and we can cast it from spell storing, but it needs a weapon with ammo. Since gunblades aren't a thing in official 5e land, not even for absurd infinite damage moonblades, that means it is only better if the hardiest creatures around are going to be killed or almost killed by a +3 Oathbow.
If that 4% miss rate bothers us, we can improve it. A Boon of Fate on our main character takes no actions and can add 1d10 on a miss that isn't a 1, but we can only do it once in the fight. If we farm Fate cards, we need not miss so often. There's a 1 in 20 chance we get a Fates card from the deck per draw (2 cards are already permanently missing so we don't break the consumption rules), and a 1 in 6 chance we draw Fates before we have to use a safety Fate and retry. If we assume only one creature can draw per round, and that drawing takes an action, we on average draw something like 5 Fates an hour. The odds of getting at least 18 Fates (enough to ensure every attack hits) are very very very good.
