Let’s say a Way of the Four Elements monk wants to use his Water Whip elemental discipline spell to grab an horrible sorcerer standing behind a Wall of Water (from the spell found in the Elemental Evil Player’s companion).
You can spend 2 ki points as an action (as per the errata) to create a whip of water that shoves and pulls a creature to unbalance it. […] On a failed save, the creature takes 3d10 bludgeoning damage, plus an extra 1d10 bludgeoning damage for each additional ki point you spend, and you can either knock it prone or pull it up to 25 feet closer to you. (PHB, p. 81, emphasis mine)
Wall of water:
You conjure up a wall of water on the ground at a point you can see within range. You can make the wall up to 30 feet long, 10 feet high, and 1 foot thick, or you can make a ringed wall up to 20 feet in diameter, 20 feet high, and 1 foot thick. The wall vanishes when the spell ends. The wall’s space is difficult terrain. Any ranged weapon attack that enters the wall’s space has disadvantage on the attack roll, and fire damage is halved if the fire effect passes through the wall to reach its target. (EEPC, p. 23, emphasis mine)
How would this work? Would the wall deflect the spell, or cause both sources of water to merge in any way? Would the whip pass through it freely? If so, how would the “difficult terrain” affect the distance over which the target is pulled, if at all?