Yeh level 3 is pretty good, linear hallways keep the pace up and makes use of those worm men well since they are close range foes, and the boss is really cool too(didn't want to spoil that). Level 5 has a good dense misty forrest section for its second half, level 4 has a very short tunnel section but outside of those sections it's all absurdly open spaces, which doesn't lend itself too well to such an arcade like shooter. There's no real design to most areas it's just bizzarly spacious outdoor areas, telling that they had no money/time to make this game.
I'm not sure about iframes on element shot animation but I know that the larger spells(meteor, lightning and tornado), raven and sacrifice(revive npc) spells have iframes, so it's likely they all have iframes. I do really like how each gun has its own element, wonder if it effects damage against certain mobs, I know soldiers burn from machin gun element, only used the cannon's light shot which makes it more powerful and accurate, I've yet to test shotgun or gatling but the shotgun wind sounds good, shame the levels are too open mostly to make use of the shotgun.
Machine gun is by far the most versalite gun, best range/damage/speed/clip overall, all the others are very situational, I haven't got the gatling gun yet.
Best spells are by far lightning for disposing tanks(and choppers, maybe use tornado for same purpose), raven's panic for distracting grunts, willpower(you need to have debris for this and aim well for it to work), ancient wall(I seldom used only against snipers but I know it's useful for higher modes) and element shot for buffed up guns. Rose spears seems useless as by the time it's activated you could have killed everyone but I've not experimented with it yet.
Survival rating is how many npc's you saved, so if you see them in a kneeling down state then use sacrifice on them, I didn't pay attention to level results but I got an S for most stages so I've done something right. Accuracy and kill count are also factors, so search all the alcoves of maps for extra goons lying around.
I've found this:
https://www.gamefaqs.com/xbox360/931220-bullet-witch/faqs/48604