To achieve this you will need a few things and a map set up to allow this.
I created and uploaded a created map so feel free to use that. If for some reason my map is not available wheter it be that there isnt crossplay between console and PC or network is taken down I will tell you how to creat the map. Map Creation
First thing set the map to have no geopanels.
Now you want your landform. I use Two "Square 3" Landforms and place them side beside symmetrically and the use
to raise the Floor 7 times. You will want to put your base up there.
Next we want a low landform. Use the landform "Double 2" and place 3 vertically against the tall structure.
Now you are going to need to buy Ally Boost + 50% and Def -50%. I cant recall the costs but just complete User maps daily to earn CP
You want to plant your 50% Ally Boost Blocks in a Cross pattern and have 8 of them stacked on top of each other to be flush with you high landform. Now 3 squares forward across to the other Square of the Double Landform and place 8 Def -50% Geoblocks on top of each other.
On top of those blocks you want to put a LVL 1 Character, I recommend a LVL 1 Healer and have her face away from the High Landform.
So you show now have a Tall Landform with Geoblocks in a Cross formation that you will be able to walk onto and 3 squares away your enemy on top of 8 DEF -50% blocksHitting for 10,000,000,000
You will need the following:
-Level 9999 Desco and Prinny Kurtis
-I had Desco's Aptitudes around 150% and +9 Yog Sothoth
-Equip her with 3 maxed Trapezohedrons and a LVL 300 Crowbar (Can be stolen from Pringer X).
-Beast Master with maxed out Dominate Skill - Character LVL doesnt matter
-The following maxed Innocents: Professional, Guardian, Monter hunter and Gladiators
Firstly you want to put innocents onto the items. Make sure you have the Guardian, Professional, Monster Hunter equipped on an item. Then just fit however many gladiators you want to max out Attack. Your other Characters equipment does not really matter.
Load up the Map.
Send out Desco and Prinny Kurtis. Fuse Prinny Kurtis with Desco to make a big Desco. You should still have Max attack. I used a Magician to put the enemy to sleep. I dont think that matters though. Now bring out the Beast Master and have it go to Desco and use Dominate. Make sure you are able to get to the Cross platform of Geoblocks within 3 turns which is the duration of Dominate. Use +9 Yog Sothoth with all empty tiles showing on the lower floor except the one hitting the enemy.
Execute and the achievement should unlock. You will get an extra bonus to attack because you are attacking the enemy from behind.