Only going to reply about MP issues with casters since the only class I play is THM and I'm not 50 to discuss dungeon parties.

First, I have been soloing R44-49 beastmen around the world nonstop with absolutely zero downtime whatsoever. I never run out of MP, so either CON is entirely different or people just aren't adjusting. Use Stygian Spikes, Siphon MP on cool down, maximize Tranquility usage by using it as soon as you dip low enough as to not waste any of it, and use your spells sparingly.
Look at the MP cost of your spells and weigh their usefulness in situations and use spells from other classes if you have them available. I use Rampart a lot, as well as Profundity and Spiritbind. In addition, use all of your free attacks (Spirit Dart, Elemental Shroud, Nature's Fury) to built TP to regen even more MP. Also, don't run long distances in active mode, switch into passive mode between battles, the small regen helps. I usually use Profundity with my Dart to generate the extra TP, and I try to use it when a mob is around 20-25% so the TP attack will kill it and cool down will be back up by the time I engage the next mob.
You can check my post history, I was having one hell of a time trying to solo the first couple days of the patch, and I won't lie it is significantly harder now, but I honestly feel the current MP costs are OK. They seem wtf high because they were so low before, but once you adjust, they aren't that bad. Just make sure you're not overhealing or pulling multiple mobs. And Stygian Spikes gives you constant MP regen in combat.
The only thing I would really like to see changed for casters is a lower cool down on Dart. Archer has a short cool down for their attack, mages should have similar treatment. With a shorter cool down the people having MP issues should have an easier time because mobs die faster and more Radiance/Damnation = more MP.