For Creative Mode:
1. Go to "The End"
2. Spawn Endermites
3. Switch to Creative [If not in creative mode already]
4. Endermen attack the Endermites
For Survival Mode:
0. Use a world where the Endermen cannot teleport away.
1. Spawn at least 10 Enderman in an enclosed 4x4x4 area(using barrier block for viewing) where they cannot see the player.
2. Spawn at least 30 Endermites where the Endermen are in the enclosed area.
3. Be in survival mode and stand where the Enderman cannot see you.
4. Endermen will attack the Endermites.