Long story for forum, check reddit or YouTube for specifics. Short story, the fireteam is separated, 3 of them will be teleported to other world, they need to get the relic and bring it back. The rest of the team need to keep the portal open for the ones that was teleported be able to return. There are two portals you have to activate the correct one based on the location of the teleported team. After that the relic holder will create a shield and all the team will jump in and start shooting. You have like10 sec, before the shield goes down. And repeat the process until the Done.