It keeps killing me and shooting seems to do no damage

Assuming you mean the boss from the base game, not the Addon:
Once the boss appears, make a run for the platforms to its left or right. From those platforms you should have a good shot at its "mouth" (it's on its top - in between all the tentacles. Shoot it until the tentacles fall down. From there you have just a second or two to jump in its mouth and then: shoot its heart.

thanks, that worked