94 Defense:
I've been mixing it up between throwing red chins at skels, camping bandits, dagannoth sentinels @ Dominion Tower, and camping armoured zombies. Thoughts based on a few runs:
1. Skels = fastest XP, highest cost
2. Bandits = middle XP, middle cost
3. Sentinels = low-middle XP, low cost
4. Zombies = low XP, GP gain