I'm not sure if this is normal behaviour, but after you kill the giant colossus and get past her first "invulnerability" phase (i.e. you kill those 4 slimes when she's standing in the middle), her health bar remains in the "invulnerable" state forever.
I was soling this boss for quite a while, and I just kept killing the adds over and over again, seemingly forever. After about 20 minutes I reset the boss and googled it, and it would seem that this boss has already been "fixed" before, since the youtube guildes I found weren't the same as my experience (her HP went down and you could see her health bar... these were from 3 years ago).
Then I found stuff on the forums (also from years ago) complaining about her and saw she was since fixed... But I still couldln't find an updated guide, and I couldn't figure out what I was doing wrong.
It wasn't until much later that I realized that her HP was indeed going down... but that the bar above her head remains the solid white "invulnerable" bar forever, leading me to believe that attacking her was a waste of time. So I just kept killing her adds, since I believed (wrongly) that more mini bosses would eventually spawn, according to misinformation I had found elsewhere online.
It wasn't until I saw a couple other players show up almost a half hour later who just started nuking the supposedly "invulnerable" boss that I saw that the huge "boss health bar" at the top was indeed lowering and I could actually attack the boss.