A living Entity is one that has health and that can die.
That means Monsters, Animals but also IPlayers.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.entity.IEntityLivingBase;

IEntityLivingBase extends IEntity. That means all functions available to IEntities also are available to IEntityLivingBase.

ZenGetter	Return Type (can be null)
ZenGetter activeHand	Return Type (can be null) IEntityEquipmentSlot
ZenGetter activeItemStack	Return Type (can be null) IItemStack
ZenGetter activePotionEffects	Return Type (can be null) List<IPotionEffect>
ZenGetter AIMovementSpeed	Return Type (can be null) float
ZenGetter arrowsInEntity	Return Type (can be null) int
ZenGetter attackingEntity	Return Type (can be null) IEntityLivingBase
ZenGetter canBreatheUnderwater	Return Type (can be null) boolean
ZenGetter health	Return Type (can be null) float
ZenGetter isActiveItemStackBlocking	Return Type (can be null) boolean
ZenGetter isChild	Return Type (can be null) boolean
ZenGetter isElytraFlying	Return Type (can be null) boolean
ZenGetter isOnLadder	Return Type (can be null) boolean
ZenGetter isUndead	Return Type (can be null) boolean
ZenGetter lastAttackedEntity	Return Type (can be null) IEntityLivingBase
ZenGetter lastAttackedEntityTime	Return Type (can be null) int
ZenGetter lastDamageSource	Return Type (can be null) IDamageSource
ZenGetter mainHandHeldItem	Return Type (can be null) IItemStack
ZenGetter maxHealth	Return Type (can be null) float
ZenGetter offHandHeldItem	Return Type (can be null) IItemStack
ZenGetter revengeTarget	Return Type (can be null) IEntityLivingBase
ZenGetter swingInProgress	Return Type (can be null) boolean
ZenGetter totalArmorValue	Return Type (can be null) int
ZenGetter moveForward	Return Type (can be null) float
ZenGetter moveStrafing	Return Type (can be null) float
ZenGetter moveVertical	Return Type (can be null) float

ZenSetter	Parameter Type (can be null)
ZenSetter AIMovementSpeed	Parameter Type (can be null) float
ZenSetter arrowsInEntity	Parameter Type (can be null) int
ZenSetter health	Parameter Type (can be null) float
ZenSetter lastAttackedEntity	Parameter Type (can be null) IEntityLivingBase
ZenSetter revengeTarget	Parameter Type (can be null) IEntityLivingBase
ZenSetter swingProgress	Parameter Type (can be null) int
ZenSetter moveForward	Parameter Type (can be null) float
ZenSetter moveStrafing	Parameter Type (can be null) float
ZenSetter moveVertical	Parameter Type (can be null) float

• boolean attackEntityFrom(IDamageSource source, float amount) → Does something...
• boolean canEntityBeSeen(IEntity other);
• boolean hasItemInSlot(IEntityEquipmentSlot slot);
• boolean isPotionActive(IPotion potion) → Returns true if the goven potion is active
• boolean isPotionEffectApplicable(IPotionEffect potionEffect);
• heal(float amount) → Heals the entity by the amount given
• IEntityAttributeInstance getAttribute(String name) → Returns the given Attribute
• IItemStack getItemInSlot(IEntityEquipmentSlot slot);
• IPotionEffect getActivePotionEffect(IPotion potion);
• void addPotionEffect(IPotionEffect potionEffect);
• void removePotionEffect(IPotion potion);
• void clearActivePotions() → Removes all active potions from the Entity
• void knockBack(IEntity source, float strength, double xRatio, double zRatio);
• void onDeath();
• void onLivingUpdate();
• void setItemToSlot(IEntityEquipmentSlot slot, IItemStack itemStack);
• void resetActiveHand();
• void stopActiveHand();