Diabolos is super easy, just need 1 player doing the door duties, remember the pairs and mark them to help memorize it (door icon appears above it for short while though). Then open the 1st door and make sure it stays open before Diabolos cast that heavy AOE, sometimes that 1st door can be force-close by one of Diabolos attack. And when it started casting that huge AOE, run and open 2nd door. Basically just that.

When I get Lost City, I usually do the door duties cause it's kinda fun. ^^