Make a hole where there is no water. Make sure it is not so deep to where it kills the mobs, since this is an XP farm, not an item farm. Clear out an area down below where you will stand as you attack the mobs. By the way, placing a torch on the spawner until you're ready...
Mob spawners, like my skeleton farm around a spawner, works great though. I attached (uploaded) the world, with the player right below the mob farm. Sadly my world is 12mb big, so I uploaded it somewhere else:http://www.ta-sa.org/files/20170508_mob_farm_spawn_problem.mcworld ...