I think the random monster spawn are not too hard, just as they should be in impossible. In fact impossible is still somewhat easy I find.
This was on an XL? world, with the maximum number of great mages (Largest possible settings basically). Normal difficulty. I'm... 234? turns into the game.
I beat exiled mode.
Allied with all of the other mages.. who were huddled in thier starter worlds, being pummeled into submission by random AI Spawns.
As far as I've noticed the players problem with taking out spawns isn't as large as the AI's ability to do the same. I am currently playing my first playthrough on exiled mode(normal) with 4 other mages. Both of the ones I've come across have been struggling to survive. So far I've managed to only lose one unit through out the game, while playing as the humans. Though the tormentors really made me go on the defencive when they spawned in a group of 3, and I've only reached tier 3 as far as worlds go.
As the game goes on you find you have to keep tons of units behind, constantly clearing the worlds you've already cleared. It feels almost like negative progression and it's really annoying and not how I'd like to play. I would hope they come out with a change for it!
I'm on turn 500ish right now, and I've noticed the slowdown and the drag of having to clean up the backside. But more than just having to clean up everywhere, the low ideal city cap on the large map is getting on my nerves. I can't build more economic powerhouses/unit factories because that would drive my unrest up and cause me to need to change my attention from fighting the enemy to fighting annoyed civilians. And not having more powerhouses means my army will eventually hit max size, asside from units that purely cost mana/gold. Slowing down my progress even more.
One trick, that still is a lot of a hassle, is to just turn every hex you don't have buildings on, plan to build on in the foreseeable future or use as a connection, into mountains. It is a slow, expensive, painfull and tedius process, but it will keep some of the spawns off your back.(if things really get on my nerves too much, that's the method I am going to resign to, probably just use meta-telleport to get everything to the frontlines)
I just have to ask, but how on earth are you still playing at turn 500? That seems like forever-long. My I beat huge/challenging in 169 turns and large/impossible in 160 turns. Like, more than 3 times that long I can't even imagine what each of your turns is like... do you only have a dozen units or something?