This is most easily done after you have completed the game once and have collected Lv 3 on most of all of the souls.
Play on Casual difficulty.
Best if done with a friend while having them be the host.
Don't rush the fight. Doing too much damage too quickly will push the boss through the phases too fast and you'll have a ton of the crystals all over the screen that he will blow up. The 3rd set of them is bad enough on it's own without mashing the others into the mix as well.
Have your friend play War, while you play Strife. As the host the bosses usually focus the host of the game which means he will stay on war most of the fight. Try to keep the boss against one of the outer walls.
Do not get hit by the fire trails that the boss leaves on the ground as these will do a tiny bit of damage but be enough to cause you to fail.
Shoot the boss from as far away as you can while keeping him on the screen so you can see his attacks.
The first phase he will just do small sword attacks and occasionally target one of you and dash at you and drop crystals on the ground in a small explosion.
At around 80% health he will grow his sword with crystals and the blade will be much larger. His attack range will be much larger than previously but still very telegraphed. Just wait till he lunges toward you and then dash as he comes to a stop and the attack will miss you.
At around 60% he will go to the next phase. Now he will swing twice with his melee attack in rapid succession. You will need to dash twice to not get hit.
Around 30-40% remaining health he will move to the next phase where he will gain a 3rd attack in his melee combo. The last attack will be aimed at the person he was swinging at and it will shoot a flame across the ground that will create crystals when it hits the outer wall. Just use strife's third dodge in order to bypass this attack. It is much easier to dodge since it is a straight line and not a sweaping melee swing like his other two hits.
On phase changes and in other parts of the fight as well he will spawn a bunch of crystals around the room. Dodge these as best you can. When he only puts down 4 or 5 crystals it is really easy. When he does the ones where there are 10 or 12 crystals you can run out of space fast. This is when it is good to keep the boss near the wall. Usually there will be a safe spot in the middle of the room when he spawns the massive pile of crystals. If not in the dead center, then the safe spot will be close to the center. Sometimes you can be safe around the outer wall as well, but there is a lot more RNG involved in those safe spots.
Use shadow clones as often as you can to help DPS and just stay so that the boss is at the edge of your view so you can always see what he's doing. Let War do most of the fighting and tank the boss for you and the achievement should be fairly easy with some practice.