If slugger is using the 1.8 craftbukkit built using the spigot process- then that version of that essentials will work with that version of bukkit.
If he's using spigot it will work.
If he's using Sponge, Rainbow they may have their own preferred permissions software. Trident isn't really ready yet.
If he's using the 1.8 from august 2014.. then who knows.
So... where did you get the 1.8 version you are using from?
Currently, the most stable platform is spigot 1.8.
In a few months, perhaps sponge or rainbow will be there.