make a 2x2 platform with hoppers in the floor feeding into chests then build up walls 23 blocks on all sides covering the hoppers with slabs. once you're at the top build the shoots for water to push the mobs down the hole by making 4 2x8 platforms on each side, the 21rd block of the wall being the 8th block of the platform. then give the platform 2 block tall walls so its equal with the other walls and then build a floor. you should now have a 23 block tall mushroom with a 18x18 top. then give the platform 2 block tall walls and a roof making sure to light up the inside while you work. then place ladders or trap doors on the top of the 2 block tall walls of the kill platforms so mobs think they're full blocks and walk over them. then put 2 blocks of water at the end of the platforms on the opposite side of the drop and remove the lighting and go to the bottom to collect xp and loot after killing mobs
