Go underground and light up as much as you can find. Light up the surface, too. Just to be sure nothing can spawn at any time.
Of course, you don't need to be perfect. I've lit up about 90% of the surface and just about 60% of caves, but my mob grinder I built a few weeks ago is doing just fine.
I would recommend you make the inside 2 and a half blocks tall using slabs under the normal roof, though. Endermen have double the health as other mobs, and they can teleport, which would most likely ruin your slaying plans.
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Watch out for the crabocalypse. Some say the day will never come. But it will.
Feel free to drop by for a chat whenever.
If you'd like to talk with me about other games, here are a few I play.
Team Fortress 2
Borderlands series (Borderlands 2 is my favorite game, ever. TPS combat is a lot of fun and makes up for the lower-quality story, in my opinion)
Elder Scrolls series
Warframe (IGN is something like That_One_Flesh_Atronach)
Pokémon series (HGSS forever)
Left 4 Dead 2 (Boomer files always corrupt though)
SUPERHOT (SUPERHOT is the most innovative shooter I've played in years!)
Dead Rising series (Dead Rising 2 is one of my favorite games, and the 3rd was a lot of fun. 1st has poor survivor AI and the 4th is bad)
Just Cause series
Come to think of it, I mainly play fighting-based games.