1 vs 6 monster

Mite Skirmisher - Siren Charm (Siren Beguiler)
If nether, max speed.
Boots with as many disables as possible(stun, frozen, snare, ect.)
Chaos Mage
Key thing is being able to go first, Attack the monster who is going after you, that will cause you to go after that monster however since there is a very high chance of the enemy being incapacitated it will lose its turn and the mite gets to go again. Continue to attack whatever monster is going next. The main counters I can think is the ones that inflict status effects on attack, so chastity, black hole halo, actually a lot of the on death abilities would still cause problems.

Is there any other creatures/artifact combo that can effectively 1 vs 6?

A Mummy Lord with any damaging counter effect would be able to keep itself effectively invincible with Death Beckons while killing enemies with counter damage.

I haven’t tested this personally, but it seems to me like a Carver Shadowdancer with the Wild Stabs enchant would make itself an illegal target for the stabs while invisible, which would let it pretty much autopilot any enemy team without blinking.

A Skeleton Gunslinger with Siren Charms could Stun its target so long as it is in the front row, causing it to skip its turn, and then take the next to repeat the process, in the process likely accruing heavy stuntimes for other creatures in the enemy party.

Very simple counter, Shell.

To your other question, fallen carnage (midnight feast) can easily 1 vs 6 because it gets a new turn every time it kills en enemy with an attack.

The ultimate problem with a 1v6 build is that it can’t really go too far, since sooner or later the enemies stays will simply obliterate you (not including abilities that counter your creature regardless of stats)

Those are really clever choices though… makes me wonder about good 2 and 3 composition teams, which can give a bit more flavor to a sub-6 creature build.

Very simple counter, Shell.
Chaos Mage

It has been a while since I have been a chaos mage, but IIRC shell gets checked before the chaos mage ability is applied.

Shell beats Abolish, yeah.