The Great Goblin, Amoeboid Changeling, and War Elemental

Prerequisites

• The Great Goblin on the battlefield
• Amoeboid Changeling on the battlefield
• War Elemental on the battlefield

Steps

1. Activate Amoeboid Changeling by tapping it, causing War Elemental to gain all creature types until end of turn
2. Put a counter on War Elemental
3. The Great Goblin's first ability triggers, as War Elemental is a Goblin, Orc and Army due to Amoeboid Changeling, dealing 2 damage to an opponent
4. War Elemental triggers, putting a +1/+1 counter on it
5. Repeat from step 3

Results

• Infinite +1/+1 counters on a creature
• Infinite damage