Crystalline Crawler and Cryptic Trilobite in hand.
Shaile and Umbral Mantle on the battlefield.
Shaile does not have summoning sickness.
Umbral Mantle attached to Shaile.
({6} magic symbol) available.
Steps
Cast Crystalline Crawler by paying ({4} magic symbol) .
Cast Cryptic Trilobite by paying ({2} magic symbol) .
Activate Shaile by tapping it, putting a +1/+1 counter on Crystalline Crawler, Cryptic Trilobite and any other creature that entered the battlefield under your control this turn.
Activate Crystalline Crawler's first ability by removing a +1/+1 counter from it, adding ({U} magic symbol) .
Activate Cryptic Trilobite's first ability by removing a +1/+1 counter from it, adding ({C} magic symbol) ({C} magic symbol) that can only be spent to activate abilities.
Activate Shaile by paying ({3} magic symbol) and untapping it, giving it +2/+2 until end of turn.
Repeat from step 3.
Results
Infinite +1/+1 counters on other creatures you control that entered the battlefield this turn.