When last I approached solo monsters, I dealt with "status effects" as "give them a save to resist."
But players don't like seeing their effects go entirely to waste. Instead, I now endorse the following rules:
For any solo that gets two full rounds worth of actions each round (and increasingly, I think that should be the norm), any "short duration" effects end at the end of each of their turns. Thus, your ENT immobilization will in fact immobilize the solo, but only for one of its two turns. This is thus comparable to immobilizing an elite, rather than an entire encounter-so it's still a big change on the encounter, but it's substantially harder to totally lock the encounter down.