(My suggestion would be to focus on smaller groups and/or multiple enemies/add spam so that focusing all your firepower on one target becomes impractical, but if there's one thing I know about SE, it's that they love to design "One Giant Mob" encounters.)
This is mainly the real issue, and why I've long been an advocate of both stronger mob AI and environmental conditions to take into account. We have 2 examples of "don't stand there" type fights with Diabolos' tiles and the final WotG fight. There are mobs out there with modes, but I wouldn't call them the most intuitive or flagging of what mode is. Omega stands out (literally) as a smart mode design between two and four legs telling you what it is and can do at that moment. I also liked the vampyr mob in Einherjar that split into multiple mobs. Only problem it had was you could literally zerg it before it could.
Stuff like that is just scratching the surface, though. I also want to say that until they actually get true instancing going, if ever, that we're also going to be limited in what we can see of encounters.