Mob grinding is probably (by my standards) the best way to farm xp as it's limitless. But it's difficult to build and maintain (i.e Raiding) I have one and it's very good I can get 30 levels while I wait by making this tutorial So This will tell you how to make one as I can't put pictures as I'm idiot (kinda). But I hope this works for you and good luck NB: This only works on: Zombie, Skeleton and Spider (If you make it well).
A FREAKING MOB SPAWNER!!! (U MUST HAS DIS! and I know it's hard but how else you gonna do dis)
A lot of blocks
A durable or many pickaxe(s)
Water buckets (5 is best)
Tier 2 + (Optional but useful as Lifts make life so much easier)
Mob Spawner must be at least 26 blocks above bedrock (I mean the spawn itself not the place it is in)
Step 1: [/b][/u]Once you've found the Spawner, LIGHT IT UP! This makes life so much easier , But also block all entrances to the spawner so no other mob or thing can get in. Claim it if you want but be careful when you do as it will be seen by other players if your close to an outpost.
Step 2: Dig 2 blocks underneath the spawner, have a 5 block width and a 9/10 block length, this code diagram will give a brief example whats going to be in it:
WWWWW Key: W= Water blocks, |= 7 block water flow, V= 22 Block drop either with a mouth with 1 length or 2
|||OO||| OO= Mob Spawner, 2 blocks above the flowing water
This is the structure for the main mob spawning area. Block this off but have a viewing point of it and don't light it.
Step 3: Build a small sized area beneath the fall which has an area which leads to the mob spawner. Block the small area but so you can hit the zombies and baby zombies can't get out. But also create an entrance so you can get in and collect missing xp and Rotten flesh which can be sold in stacks. If this is done properly you won't get hurt and you won't loose much.
Step 4: ENJOY! This will give you unlimited xp if it has a 22 block fall so you can just punch zombs to kill them and get xp but none die down the fall. Mine was too close to bedrock so I have to use stone sword to kill them . I may make a video on youtube showing you mine and how I do it.