Activate Necrotic Ooze using Heroes' Bane's ability by paying {2}{G}{G}, putting at least five +1/+1 counters on it.
Activate Necrotic Ooze using Crystalline Crawler's first ability four times by removing four +1/+1 counters from it, adding {2}{G}{G}.
Repeat

Infinite +1/+1 counters on Necrotic Ooze

Infinite colored mana

Necrotic Ooze

Heroes' Bane

Crystalline Crawler

Activate Crystalline Crawler using Heroes' Bane's ability by paying {2}{G}{G}, putting a number of +1/+1 counters on Crystalline Crawler equal to its power.
Activate Crystalline Crawler's first ability four times by removing four +1/+1 counters from it, adding {2}{G}{G}.
Repeat.

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