Perhaps I should reword it to "Creatures cannot be harmed by its owner for 3 turns" and remove the unlimited of the Great Sentinel and make it 5 turns.
That way it prevents anything that could possibly harm your own creatures.
The following would not be able to be done to your own creatures;
casting any spell on friendly creatures that could harm them (including poisoning)
targeting any creature with an ability that would kill them (paradox for example)
Virus' ability
Aflatoxin
Retrovirus' ability
Gravity Pull
Otyugh's Devour
Basilisk's Blood
Fire Bolt
Immolation
Fire Lance
Cremation
Freeze
Ice Bolt
Toadfish's Inflate
Psionic Wave
Congeal
Ice Lance
Owl Eye's ability
Shockwave
Reverse Time
Eternity's ability
Rewind
Scarab's Devour
Parasite's Infection ability
Drain Life
Liquid Shadow
Lightning
Thunderbolt
Benefits would include;
Mutation would only help creatures, not harm them
Fallen Elf's ability would only help creatures, not harm them
Pandemonium would only harm enemy creatures
Flooding only kills enemies
Rage Potion on your creatures would only boost attack
Unstable Gas only harms the opponent