here is what you will have to do:
make a 23 block high tower thingy.

make a redstone torch tower powered up by a button like that: (make them activate the pistons) (use lever to active the redstone, this is to turn falling mobs on/off)

Make sticky pistons pushing the 22 block like this:

then make a room like this which got is 2 blocks high:

Warning! it is supposed to be 8x8 (not including walls)
make a row for water to flow like this:

Also have an entrance like this:

This is a super efficent.
P.S if you play on .net and wanna make this you have to place a spawner in the middle of all or one of the squares, all depending onv how efficent you want it.
Enjoy
(for extra details contact me)

 

Redstone and piston for non donor is hard to get in .org and not legit

 

I made this before restart, and a good spawner isnt cheap

 

No need for pistons really. Good mob tower nonetheless

 

I added them so you can disable the mob spawner/gather up millions of mobs. Only trying to make it easier to get all of that mob drops