Minecraft – How to make zombies/skeletons not burn in sunlight without changing their looks

Use NBTExplorer to open level.dat of the world, then change GameRules/doFireTick from true to false.

First create a clock with the following commands:

execute @e[name=TPSlimeBack] ~ ~ ~ tp @e[type=Slime,name=SlimeKnock,rm=1,r=2] @e[type=ArmorStand,c=1,name=TPSlimeBack,r=2]

And:

execute @e[name=TPSlimeBack] ~ ~ ~ /effect @a[r=2] minecraft:resistance 1 100

Then summon an invisible ArmorStand and an invisible Slime with a size of 1 on the block where you want it to knock the player back by doing:

/summon ArmorStand ~ ~ ~ {CustomName:TPSlimeBack}

And then on the same position:

/summon Slime ~ ~ ~ {CustomName:SlimeKnock,Size:1}

This will make a Slime that knocks the players back, but that can't move out of the block's radius because it teleports back to the armour stand all the time. The player also get's a resistance effect to make sure no one loses their lives.

Edit

A much easier way to do this is just to summon a Slime on the block with the Size of 2 and NoAI added. Do this by running the following command:

/summon Slime ~ ~ ~ {CustomName:SlimeKnock,Size:2,NoAI:1}

(Note: The custom name isn't really needed, just to kill the slime when you want to remove it.)