Wall of Reflection
Taunt. Enemy recieves the same amount of damage that was dealt to the Wall of Reflection.
Two strongest enemy minions attack enemy hero.
Wall of Reflection should probably cost quite a bit more.
Consider that Shieldbearer and Goldshire Footman are 0/4 and 1/2, with both costing 1.
Based on that, we can assume each attack is equal to 2 health. Looking then at Warden 1/7 and Creeper 3/6, we can get to 0/9 for 4 cost and 0/12 for 5 cost.
There are two problems though. First is that cost per health should go up by more than the norm beyond 8 or so, since it becomes increasingly difficult to remove, even with a combination of spells and minion attacks. A mage with fireball could do 6, but you've got 6 more to do. So I'd argue an 0/12 taunt should be closer to 6 cost, but I'll keep it at 5, rounding down.
Then you've got the reflect damage. Some secrets sort of emulate this, like misdirection, which is 2 cost, or like Eye for an Eye, which is 1 cost but not aimed back at minions for field control. The problem is this effect is constant. If we say that the average cost for reflecting damage ONCE is 1.5 cost (average of 2 secrets), then I would say double that for a persistent effect, and make it +3 cost here.
So I think low-balling the cost, it should cost at least 8. When you factor in the difficulty of removal plus the persistent reflection with the requirement that you address it as taunt locks out all non-spell field control, it should probably cost 9 or 10.
The best argument I can see for it costing 8 and not 9 or 10 though is that it can be dealt with via insta-kill spells, polymorphs, hunters marks, poison minions, and silences. Of course, if your opponent doesn't have those, this thing basically makes the entire field your playground.
Onto Hypnosis, it should cost more too. Closest thing I can see is Shadow Madness with 4 cost and 1 turn control for a minion of 3 attack or less. I think, first, it should be 2 random minions, and two, it should probably cost something like 6. If you let yourself target, if should probably cost 7. Two random minions dealing direct damage to the enemy character is still very powerful. Sure, you can't remove it, but assuming you play it at a tactically prudent time (just played a boulderfist ogre or something, and only has two minions on the field), you can put a lot of damage on your opponent for very little cost.