Is there a good reference to how to get some of the accomplishments in Occult Defense? Some of them seem to be based very significantly on luck and have a very low chance of happening (or maybe it is a sign I need to do more 5-person OD instead of solo OD).
Depends on the achievement. Some are doable solo. Some are waaaay easier with more people and some need a group.
Generally stuff is easier in groups. There are also some that are designed with the expectation that you run OD a lot so you have Rng throw you a bone at some point on the way to the elusive 5k waves.
One in particular ccc-combo was easiest to get in a 5 man group as tank. You could also do sole but the chance to get all the effects at once was quite low in a single wave solo.
I was told it easier in 30+ to.
You get more mutators per wave as you get into higher waves, so it makes sense, given just how many different things you need to be hit by!