Second run on the boss and went down on phase 3 with a squad full of people. I really don’t know how this is possible it is insanely hard, can someone give me tips?

Should I try it solo again? Last time I ran out of ammo because I kept buying wall armor but it was definitely easier. It’s just the steps take longer when it’s only me, I really want the early calling card and some people apparently have been getting it without killing the boss but of course with my luck I didn’t get it. If anyone can help me with advice I’d appreciate it thanks