Xjyuk

(I have the scoreboard set up, use "Levi" for it in comments)

Example: I hold a stick. All creepers in a 6 block radius around would get Levitation until I stop holding the diamond or (Optional) they leave the 6-block vicinity.

I want to hold an item, give the set mob type mobs in my radius Levitation, but only in that radius.

Help is very much appreciated! :)

First, create a new scoreboard objective:

/scoreboard objectives add holdingItem dummy

The following commands should be placed in Repeat Always Active comand blocks:

1. 
   

   To track players holding your preferred item (I'll assume it's a diamond) you need to give all players holding a diamond a holdingItem score of one:

   
   
   

   /scoreboard players set @a holdingItem 1 SelectedItem:id:"minecraft:diamond"
   

   
   


2. 
   

   You'll need to reset the score every tick, so the Creeper doesn't go flying every time you get close:

   
   
   

   /scoreboard players set @a holdingItem 0
   

   
   


3. 
   

   Now, all you have to do is execute the command for all creepers around any player with a holdingItem score of 1:

   
   
   

   /execute @a[score_holdingItem_min=1] ~ ~ ~ /effect @e[type=Creeper,r=6] minecraft:levitation 1
   

   
   

TL;DR Edit: One-command version

Copy-paste this simplified command into a regular impulse command block and run:

/summon minecraft:falling_block ~ ~1 ~ Block:command_block,Time:1,TileEntityData:Command:/scoreboard objectives add holdingItem dummy,Passengers:[id:minecraft:falling_block,Block:repeating_command_block,Time:1,TileEntityData:Command:/scoreboard players set @a holdingItem 1 SelectedItem:id:"minecraft:diamond",Passengers:[id:minecraft:falling_block,Block:repeating_command_block,Time:1,TileEntityData:Command:/execute @a[score_holdingItem_min=1] ~ ~ ~ /effect @e[type=Creeper,r=6] minecraft:levitation 1,Passengers:[id:minecraft:falling_block,Block:repeating_command_block,Time:1,TileEntityData:Command:/scoreboard players set @a holdingItem 0,Passengers:[id:minecraft:falling_block,Block:command_block,Time:1,TileEntityData:Command:/fill ~ ~-5 ~-1 ~ ~1 ~-1 redstone_block,Passengers:[id:minecraft:falling_block,Block:redstone_block,Time:1]]]]]

It will throw a few command blocks above it (one impulse, then 3 repeat, then 1 impulse). To stop it, destroy all the command blocks.

Use /effect to add the levitation effect to mob and use @e[r=6] to seelct all entities in radius of 6 to add the effect.

So, /effect @e[r=6] minecraft:levitation 1 in a repeat command block would work giving all entities in the radius of 6.

If you want to that players are exluded, you should use @e[type=!Player,r=6] instead of @e[r=6].

There is an easier way than that although not exactly practical you can just set up a command that enchants something and feeds of that to make a mob float but that's my opinion.