Evolution Witness | Ashnod's Altar | Host of the Hereafter (and one other card)

Creature with MV<=2 that is colorless and enters the battlefield with a +1/+1 counter on it

Cards

Evolution Witness
Ashnod's Altar
Host of the Hereafter
Creature with MV<=2 that is colorless and enters the battlefield with a +1/+1 counter on it

Creature with MV<=2 that is colorless and enters the battlefield with a +1/+1 counter on it in hand.
All other permanents on the battlefield.

You have enough mana to cast the creature with MV<=2 if it doesn't have ({X} magic symbol) in its mana cost, or enough to cast it with X=1 otherwise.

Cast the creature from your hand by paying ({2} magic symbol) or less, causing the creature to enter the battlefield with a +1/+1 counter on it.
Activate
Ashnod's Altar by sacrificing the creature, adding ({C} magic symbol) ({C} magic symbol) .
When the creature dies,
Host of the Hereafter triggers, putting a +1/+1 counter on
Evolution Witness.
Evolution Witness triggers, returning the creature from your graveyard to your hand.
Repeat.
If the creature can be cast for ({C} magic symbol) or less, this combo nets infinite colorless mana, and you may cast that creature for an arbitrarily large amount of mana if it has ({X} magic symbol) in its mana cost.

This combo has been estimated to be Exhibition (Bracket 1+).