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Raise a Floppa - The Loop
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You see a deathslicer.
Notes
Deathslicers are immune to all known damage types that players can attack with. However, if someone manages to kill one of them, it will respawn immediately after dieing. Note that they are pushable.
Abilities
Damage Taken From Elements
Physical 

Death 

Holy 

Ice 

Fire 

Energy 

Earth 

Location
Deep in Tarpit Tomb, Formorgar Mines during Ghazbaran's raid.
Behaviour
Deathslicers will get as close as possible to you, attacking with its strong Melee and heavy Energy-Based Spells. A deathslicer will retreat at 20 (0.24%, deep red) health.
Strategy
Use a stronger monster such as a Demon to destroy it, when it's trapped. You can push it.