For this achievement, you need to combine two cells of power level 2 with The Middleman, who is left from where you spawn in Ramsgate.
You unlock him quite shortly into the story.
To combine cells, you require 2 cells of the same rarity (Level one, two or three)
If you combine 2 cells of the same name (so Sharpness +1 and Sharpness +1) you will get a sharpness +2.
Also, if you combine 2 cells from the same category, you will get a cell of random quality from the same category (so 2 random technique cells (1) will produce a technique cell (2)
2 totally random cells of the same level will produce a totally random result (so technique (1) and power (1) has a chance to make any (2) available.
It takes 22 hours to make a Level 2 cell, and 46 hours for a level 3. This can be expedited if you have earned the purple chips available from the "Hunt Pass" Which you only need between 20-40 of dependant on the time remaining for cell fusion. (You can also Exchange two random level 3 cells for a new random level 3 cell, which takes 74 hours too)