Okay, first thing's first, never cluster. All of your team should be spread out, you should have at least 1 medic (preferably 2), and you should not waste all your best ammo in the first phases. Save those for phase 3/4. Also, he's weaker on his head and back, so aim for those. And run from gas. If he throws gas or nades, just run away, forget about shooting him until you're safe. 2 nades will kill you at full health / armor, 1 will get you down to like 30 health 0 armor on Hard.

Phase 1: just shoot him nigga, he's not that tough Phase 2: He can now do giga nigga melee attack. If he starts doing it to you, pray an explosive stops him, or start parrying, because if he gets right up to you then you can kiss your ass goodbye. Otherwise shoot him. Phase 3: He'll start doing melee more often here. If someone else is getting meleed, use your nade on him, since that's one of the few ways to get him stunned for a second and not kill someone. Phase 4: Kite kite kite kite kite. He'll almost entirely be in melee mode now because he's pissed off. Just kite him. If he's going after you just run away until he gets close then try to parry his attack, but trust your teammates to help.

Also, when he grabs someone, don't be that one retard that shoots him or throws grenades. Doing damage to him while he's draining someone just makes the draining go faster, does 0 damage to him, and gives you and your team less time to heal your teammate. What you should do instead is heal the guy getting grabbed, and reload your weapon(s) for the enxt phase. DO NOT SHOOT HIM IT IS A WASTE AND ACTUALLY HURTS YOU, I don't know how much I have to fucking tell that to random idiots I play with.