Ok campaign stuff here, don't read unless you want to be spoiled.
I swear chapter 6 is impossible after trying it for about 2 hours now at least.
I've finally gotten the waves down pretty easily (I'm using the rifle (which is so damn awesome) and a staff of war for the health bonus), so I figure I'll throw my 2 cents as to how to beat them to add to what nenjin put down earlier. The rifle makes it a lot easier, but a lot of the stuff I'll say will apply to other weapons and staves.
I've found that this has really drilled how good the resistance spells are, it'd be impossible without them.
First wave can be gunned down easily, before the stage changes fire off a heal (staff of war) and line up haste.
Second wave, fire off the haste and a warp time, run around the bottom druid (to get away from the tree) and you can gun him down before the tree gets to you.
By then the other druid will have started rain, so you'll want to get up water and fire resistances. Then this bit is unforunately a bit of luck, as you need to get in a good position to mow the other druid down without an unlucky knockback. From here on you should try to avoid laying the last blow each round until your health is at full, so you can use haste before the next round starts and line up warp time.
Third wave's the hardest by far, but the strategy here works for all the rounds after. I run for the wall and set up a rock barrier around me (4 earths+1 shield) to give me time to set up lightning and arcane resistances, otherwise the 3 guys will demolish you in seconds, reset the rocks if you need to heal/reset resistances/haste. From getting knocked around by these guys, I've found the best knockback spell is 4 waters+1 earth, provided there isn't anything in the way one of these will knock anything it hits off the stage if fully charged. keep behind your rock wall charging a spell and fire it off when the enemy breaks part of your wall, then bring the wall back up and try again. Warp time isn't really needed once you get set up
The last 3 waves are by far easier than the third one, as you wont get knocked around by much except crossed beams. Waves 4 and 6 can be dangerous if you don't keep lightning resistance up, and wave 5 you'll start in water, but otherwise it's rather easy, either knock them off or buff up and gun them down.
My problem is the big guy himself, he just hits way too hard, and I can't see how you can beat him in single-player without absurd luck. With warp time+haste up, I can only barely keep myself alive against the spirit's lightning spells even with resistance up (and if it drops, you're flat out dead, I completely gave up after getting one-shotted by a lightning bolt spell (unavoidable) out of nowhere), and I just don't have the time to throw anything at him, and it doesn't help he has a shield up half the time (what's the best attack to throw at him? I'm tossing up between shooting him (more damage but can't line up another spell) or boulders).
Anyone who's actually beaten the guy 1v1 got some advice to give for it, it just seems too dependant on everything going perfect for way too long to be realistically beaten, it's simply stopped being fun.
On another note, anyone tried the challenge mode yet? It's so much more fun than the campaign imo (depending on what you get you really have to use everything possible to survive, and tactics are far different due to the enemies coming from all directions), though once you get a summon spell it gets kind of rediculous, my goes, once I get summon elemental devolve into swarming the waves with a rainbow of different coloured elementals.