Cayth, Famed Mechanist | Mana Echoes | Singing Bell Strike

Cards

All permanents on the battlefield (
Singing Bell Strike attached to
Cayth).
You control at least one creature token.
You control at least seven creatures that share a creature type with the creature token.
Cayth does not have summoning sickness.
({2} magic symbol) available.

Activate
Cayth by paying ({2} magic symbol) and tapping it, creating a token copy of a creature token you control.
When the creature token enters the battlefield,
Mana Echoes triggers, adding ({C} magic symbol) for each creature you control that shares a creature type with it.
Activate
Singing Bell Strike by paying ({6} magic symbol) , untapping
Cayth.
Repeat.

You may begin the combo with less creatures that share a type with the token, but you will need additional mana to do so until you begin to break even from
Mana Echoes' trigger. Once you have infinite mana, you may instead use
Cayth to get infinite proliferate.