On an unrelated note, I've been making some creatures for a potential map I was thinking of making. The idea is that the only way to get any core resources (wood, stone, iron, obsidian, that sort of thing) was to kill special mobs found throughout the map. They've all got some special bonus ability as well to spice things up a little.
/setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {EntityId:ThrownPotion,SpawnData:{CustomName:"an Ironforged Sentinel's Electric Burst",ownerName:an Ironforged Sentinel's Electric Burst,Potion:{id:potion,Damage:16418,tag:{CustomPotionEffects:[{Id:1,Amplifier:1,Duration:200}]}},Riding:{id:"ThrownPotion",CustomName:"an Ironforged Sentinel's Electric Burst",ownerName:an Ironforged Sentinel's Electric Burst,Potion:{id:potion,Damage:16,tag:{CustomPotionEffects:[{Id:5,Amplifier:1,Duration:200},{Id:7,Amplifier:1,Duration:20}]}},Riding:{id:"Zombie",CustomName:"Ironforged Sentinel",CustomNameVisible:1,Equipment:[{id:anvil,Count:1,tag:{display:{Name:Sentinel Piston},ench:[{id:19,lvl:1}]}},{id:iron_boots,Count:1,tag:{AttributeModifiers:[{AttributeName:"generic.movementSpeed",Name:"IDS Boots",Amount:.2,Operation:1,UUIDLeast:894654,UUIDMost:2872}],display:{Name:Sentinel Boots,Lore:[Spring-loaded.]},ench:[{id:0,lvl:1}]}},{id:iron_leggings,Count:1,tag:{AttributeModifiers:[{AttributeName:"generic.knockbackResistance",Name:"IFS Legs",Amount:.25,Operation:0,UUIDLeast:894654,UUIDMost:2872}],display:{Name:Sentinel Greaves,Lore:[Braced.]},ench:[{id:0,lvl:1}]}},{id:iron_chestplate,Count:1,tag:{AttributeModifiers:[{AttributeName:"generic.maxHealth",Name:"IFS Chest",Amount:4,Operation:0,UUIDLeast:894654,UUIDMost:2872}],display:{Name:Sentinel Exoskeleton,Lore:[Sturdy.]},ench:[{id:0,lvl:1}]}},{id:iron_block,Count:1,tag:{ench:[{id:0,lvl:1}]}}],DropChances:[0.1F,0.1F,0.1F,0.1F,0.1F],Attributes:[{Name:generic.knockbackResistance,Base:.25}],Silent:1,ActiveEffects:[{Id:14,Amplifier:0,Duration:20000000,ShowParticles:0b}]}}}}
They've got some nasty tricks up their sleeve, but killing a group with a stone sword and leather armor is definitely possible.