Szarel, Genesis Shepherd | Crystalline Crawler | Slimefoot and Squee (and one other card)

Cards

You control a Saproling.
Szarel has power 3 or greater or you have another creature card in your graveyard.

({1} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) available.

Activate
Slimefoot and Squee by paying ({1} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) and sacrificing a Saproling, returning it and up to one other creature card from your graveyard to the battlefield.
When
Slimefoot and Squee enters the battlefield, it triggers, creating a 1/1 Saproling creature token.
Activate
Phyrexian Altar by sacrificing
Slimefoot and Squee, and the other creature returned in step 1, if applicable, adding ({B} magic symbol) , ({R} magic symbol) or ({G} magic symbol) for each creature sacrificed.
When
Slimefoot and Squee and up to one other creature are sacrificed,
Szarel triggers that many times, putting a number of +1/+1 counters equal to
Szarel's power on
Crystalline Crawler for each trigger.
Activate
Crystalline Crawler up to three times by removing three +1/+1 counters from it, adding up to ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) .
Repeat.

This combo can put infinite +1/+1 counters on each creature you control except
Szarel so long as its power is greater than the mana needed to activate the creature's ability from your graveyard.

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