I'm playing through the new WM content on PotD difficulty using a a full new character and a full party of hirelings. I wanted to do a second play through of the original game anyway (my first was on Hard, very different). The biggest change for me has been the NUMBER of mobs. See, even if my tanks COULD hold onto every monster they engaged, there would still be 3 more dood's trying to flood my backline. Here's my combat tips for dealing with it.
1) use choke points. Never fight in a big open area, you get swarmed and ownd. Use natural chokepoints to funnel enemies up against your tanks (I have a Monk and Barbarian, both oriented for control/debuffing)
2) use whispers of treason. You know theres a cipher in your party, and you can cast whispers right at the start of the fight. My strat is to charm the biggest stupidist looking guy, but try to pick an enemy who is between you and more enemies. He'll turn around and you'll have three ogre's punching each other for 10-15 secs.
3) use ALL YOUR BUFFS. If you do not start every fight by casting 2-3 buffs and 1-2 debuffs you will die. Anything that effects deflection, accuracy, resolve, perception, or recovery speed. Monsters in PotD have bigger stats then you, and if you don't use your spells to balance it out in the first few seconds of combat, some dude with rifle will crit and oneshot your DPS.
4) Paralyze and Knockdown. MOAR CC, even paralyzing some baddie for 5 seconds can be huge. It gives you enough time to heal back some endurance, or reposition your casters farther away. Switch up the type of CC also, it'll take me 3 fetid carcass casts to keep dat Ogre CC'd for 15 secs, or one repulser seal which will know down 3 ogres for just as long, each mob is more vulnerable to a different type of control.
5) Don't be afraid to disengage and run. Talents like Fast Runner, or abilities that give bonus deflection when disengaging can be GREAT for your casters (arcane Veil/Fleet foot for Wizards). When that enemy takes the disengage attack from your tank, pause the game and run your casters away! That baddie is coming for them! Figure out which guy he's running towards and use someone else to intercept of CC him so you can escape.
As far as going back to tweak previous parties? I can't speak to that (since i made a whole new party) but I'd just make sure you didnt leave Perception as a dump stat on anyone, as the accuracy is pretty beneficial. I gave my casters some hide armor (wanted that freeze DR) to protect from the WM mobs always throwing balls of cold at me, and made sure to have very careful positioning before each fight starts.
Anyway those are my thoughts. Happy Adventuring!