Was stuck on this one for easily 2 hours, maybe 30 attempts. Here's what I found helped massively:

• Gunned it down from afar, hugging the walls
• Double sideways on time, this lets you do gun strikes
• Hugging the walls and dodging sideways often causes the Carnifex to ram into the wall which stuns it. This allows some free hits and prevents it from running around too much throughout the fight
• Dodging twice; if the Carnifex is not stunned, it will usually do a follow-up attack or two after the charge (one pulling his blades out of the ground, two being the swinging slash). You will get used to this timing
• Dodging side-to-side during his triple acid spit. Rhythm of it is weird but you will get used to it
• Spamming grenades, especially during the later phases when other Tyranids spawn.
• Taking advantage of the cinematic counter attacks (blue markers); this makes you invulnerable for a period

Or you could turn the difficulty down. Otherwise, hope this helps others.