This achievement is for beating Malignance the first time, which occurs exactly half way through the campaign... as you may have guessed from the achievement name.
The first time you face Malignance is not so tough. He is Level 4, which is the highest level you would have faced so far, but he is comparable to the few opponents you would have faced prior to Malignance in terms of difficulty.
As with all fights, you should choose two suits that you are comfortable with using, and try to complete 4+ Super Attacks in a row to deal lots of damage. I usually recommend either starting your first Super Attack immediately as the fight begins, or wait for Malignance to use a Super Attack, dodge it, and then go on the offensive.
Please either comment below or send me a Personal Message if you have any comments or improvements.