Our 3.5 gaming group just retired (semi?) a 60 - 85th level combined multiclass characters, who've all died at least once, some on many occassions. We had a psioinicist who had two psionicist classes and 20th level fighter - overpowered brutes, all, my character included, so my recent experience has been much higher than your situation. However, three players exchange the DM's hat about every month, so we don't get DM burn-out. When we DM our own PC are away and gain no experience until we're a player once again. We've kind of developed a formula for building a challenge that I think can work at your lower "high level" game - and I know that Rolemaster is not as "hi powered" as D&D 3.e+.
I develop a side adventure at my turn on the seat, some goal that will take up to 4 gaming sessions to achieve, you can do it in one or two. For ourlevels we are saving the multiverse from some epic horror a 667th level of the Abyss that will turn the balance of power to chaotic evil, or having to go to the Far Realms to stop and multiverse ending scenario on it's home plane - big epic stuff.
Find some villian from the PCs past, bring him in on a short revenge mission against the PCs who thwarted him in the past, perhaps not ever directly involved but their past actions ending some diabolical plot of his own.
Choose one exotic hazard. You mentioned magic dead fields, that's one. Sometimes we invent exotic fogs that severely limit vision, slows missle weapons, halves the damage of area based arcane attacks - fireball, etc. Temporal anamolies, Greater Darkness, floating bits of the Negative Material Plane in pockets in the darkness, unstable terrain or the area suffers many small earthquakes. Then play a normal balanced RPG fight
Basically you need to remove some of the PCs high powered advantages, but differentiate it so not all are affected, all of the time - this let's all PCs shine above the others in small dramatic ways to enrich the story of our campaign over the course of the multi-session adventure. Every once in a while have a normal encoutner or fight unhindered by environmental conditions, so the PCs don't feel they're always getting screwed.
That way you don't have to always go Nova on one trick, make a list of possible exotic environmental conditions that could somehow, affect vision, hearing, magic suppression, magic dead, divine only, arcane only, affects missle weapons, affects saving throws, etc. When building your plot make a logical reason for your chosen conditions that fits the storyline and the villain. Have it vary, have a no holds barred fight with a lieutenant of the your villain without impaired conditions, and end with a powerful exotic condition for the final battle against the bad guy - achieve the goal, take possession of the "artifact" to return to the temple it was stolen from... then we pass the DM's hat to the next person, and come back as a player, once again.
Final note - I didn't read most of this thread and tried to leave whatever story you want to tell out of the description to apply to any high level adventure. Our campaign almost lasted 5 years, we've gained some experience at DMing high level parties, and this is generally how each DM in our group runs the games for effective, and fun high level games. The condition impairments made the job of the DM a bit easier and a touch more dastardly to making exciting month-long adventures (we play most every Saturday night, RL conditions bearing.)
GP