Ok, my rather common experience is as follows: I am a 26th lev Doc with a ql 28 Springfield Arms Home Defender. I zone into the mission, get swamped by three mobs (grey / green) --beat them without zoning and encounter a single yellow / orange later in the mission.
I've seen this with the umm... Rookie Explorer, I think, and most other nano-using mobs. This yellow is brought down to about 20% hp with conventional combat (no special attacks) and DOTs (Primitive Nano Gorger and Biotoxin MK I stacked). All the while, I am chain healing and chaining nano stims, but I can do this because I'm a paying customer and the mobs are not, right? (I'm SUCH a hippocrit!)
Sometimes, I will pull the mob out of his original room to prevent the mobs in the next rooms from aggroing on me while fighting the original yellow ([strategic] retreat after combat has started). Anyways, once the yellow is brought down to about 20% his chain-heal casting starts, and it seems like a natural regen reflex kicks in as well. I could be wrong about that last part, tho--it may have just been a heal over time nano. I noticed all of this just today (19th) after the most recent server-side patch that fixed the mission distance and locate person / object missions. Mobs just after that patch and up until this morning were perfectly balanced--for my doc.
I think the coders tried to stop the mid level mission zoners by increasing the regen for mobs. However, I believe they inadvertently pushed the difficulty up for lower level characters too much, and here we are.
Thanks for considering this, and I really hope it helps! Gimme a reply if you need -any- more research done--I'd be glad to help improve this already-great game!