A level is an exam metaphor meaning difficult, i.e harder than GCSE or standard procedures; possibly dates back to when standard entry requirement for mil pilots was just O levels.
In cloud, anti-cols off and nav lights to steady (not flash) was usual.
Also quite hard was formating on another aircraft type.
On fast jets, you might have had to do a rapid join from a tactical formation, followed immediately on joining formation by a low level abort into cloud for obstacle clearance. This was guaranteed to cause severe disorientation for the formating pilot(s). An important part of instrument flying training was to desensitise you to this disorientation. You got used to it.