For an enemy, an attack like this is very simple.
Make sure there are at least two enemies in the troop that will feature this attack. ONE MUST NOT KNOW THE MOVE.
Make a free skill. Name it whatever you want. The icon doesn't matter. Don't teach this to any actor or monster.
Make a different skill that costs however much MP or HP or whatever. Give it the same name of the previous attack. Teach this to whatever monster
you want. HOWEVER. Do not make it do any damage or anything. Make it do the following common event:
Upon the use of the attack, make it set a variable to two and cause the enemy to run away. That means the enemy with this attack always has to be in a certain spot in the enemy party. Oh well. This also limits you to only having one enemy like this on the opposing team.
Make a battle event page that runs at the end of each turn (doesn't work with ATB) and checks to see if the variable is set to zero. If it isn't, -1. If it IS zero, make the enemy reappear and use the first skill.
Another way to do this (works with actors.
You need to make 2 copies if you plan it for actors AND enemies.) is to make a state that increases your Defense to the max and guard every element and state. Make sure you set the state to disappear after one turn.
Make a variable that is called ATK or something. This is used to track the user's ATK stat. If you're using this skill for enemies and actors make two of this variable. Maybe call one ATKe so you can differentiate. Repeat this step but with a variable for Defense instead of Attack. Also make a variable called DAM.
Make a skill that costs HALF of however much you want the entire attack to cost. This skill has no effect. Add a common event that checks to see if you have the max defense state. If you do, set ATK to an actor(or enemy)'s Attack. Set DEFe to actor/enemy's Defense. Then do this:
2(ATK - DEFe) = DAM
This subtracts enemy's DEF from actor's ATK, multiplies that by 2, and makes DAM equal that.
Go onto the last(first) page and make Enemy(Actor) 1 (or entire party) take damage equal to variable DAM. After that, remove the max defense state.
If the user doesn't have max defense, add it and do nothing else.
This is a little clumsy since I just now came up with it, but it should work.
Just realized this is for RMXP. What I posted is for VX, but should be very similar on XP.