Ashnod's Altar, Putrid Goblin, and Hardened Academic

Prerequisites

• Ashnod's Altar on the battlefield
• Putrid Goblin on the battlefield
• Hardened Academic on the battlefield

Steps

1. Activate Ashnod's Altar by sacrificing Putrid Goblin
2. When Putrid Goblin dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it
3. Hardened Academic triggers, putting a +1/+1 counter on Putrid Goblin
4. Repeat

Results

• Infinite ETB
• Infinite LTB
• Infinite colorless mana
• Infinite death triggers
• Infinite sacrifice triggers