So, this is a bit of a strange one.
The attack effects are done in steps, and you do as much as you can for each step along the way.
You move the damage counters off of Mewtwo EX.
Those damage counters can not be placed on Regice because of Resistance Blizzard, so they are discarded.
Likewise, you cannot move the damage counters off of Regice because of Resistance Blizzard, so any that are on it, stay on it.
And, since you didn't move anything off of Regice, there are no Damage Counters to move onto Mewtwo EX.
The end result is that Mewtwo EX is healed of all damage.