Calus is the final part of the Destiny 2 Leviathan raid and he is not nearly as tough as he might appear. To begin this battle shoot the cup out of his hand to make him angry and cause the big guy to start calling his army. Just shoot them until Calus gets mad and warps you to alternate dimension where he is a giant floating head. Try to stay close together and make sure to pop a few health pools or bubbles to keep everyone alive. This fight is way harder if you lose someone early, so there is no need to waste any of your precious revives.
After you’ve been transported, Calus will begin to pull your fireteam towards him so have three people prop themselves up against barriers that form. The other three will need to grab orbs which will transport them back to the main boss arena. Around you are a bunch of Psions that are marked with the familiar symbols you’ve been seeing the whole raid. Have the people in the alternate dimension call out which symbols they see appear on Calus’ forehead. Those in the boss arena should kill the Psion whose symbol was not called out.
Doing so will move the barriers a bit closer so your team of three will need to keep repeating this process until Calus starts shooting skulls at the alternate dimension team. Dodge the skulls and grab the orbs that appear to have the rest of your fireteam regroup in the main boss arena. Calus can now take damage so quickly have everyone move to a single platform. From here all you need to do is shoot Calus until he becomes immune again and then repeat this process once more.
The biggest thing to the remember in the Calus fight is to have a clear line of communication between the two teams so they don’t kill the wrong Psion. Once this poor Cabal falls you can collect your rewards from the chest that spawns. Congratulations Guardian you have finished Destiny 2’s first raid!