Soulcraver gives some great tips for individual enemies, but I've seen many people recommend a slower play style for the harder difficulties, and since I managed to get 1000 on this game by running and gunning, I figured I'd go ahead and detail how I managed to get through it all. In a way, I suppose this is primarily for people who want decent scores during these play throughs, but there might be some stuff for your average player as well.
My rule of thumb for EVERY level is to spam the hell out of Ravens Panic, but you need to be mindful of the cooldown, as there's about a 5 second period toward the end of the spell where your ravens will still be out, but enemies can move around and shoot. You can combine this with Fire Bullets on your Assault Rifle to take guys out quickly, and you should be able to kill more than enough enemies to get your MP back.
Stage 1 is pretty easy, regardless of the difficulty level that you're playing on. On Chaos and Hell modes, the three guys at the beginning of the level can tear you to shreds in seconds, so you need to cast Ravens Panic as fast as possible. Go ahead and progress through the level normally, but be mindful of the guys on the porches of the houses, as you can sometimes miss these dudes initially. Casting Ravens Panic can help you pinpoint the location of enemies, as your swarm of birds will be a bit easier to spot. Your first Walnut Head (aka Brain Dude) can be taken out with a buffed Cannon (what I'm going to refer to as a Sniper Rifle), which I recommend doing. The other Walnut Heads that you'll encounter in this level should be taken out with fire bullets, as they're almost always around the corner of a building and you don't want to wait for these guys to pick up vehicles and debris. The only other tricky part of this stage is the bridge. You should hang back a bit at the top and peek over the edge because you don't really want to give the tanks an opportunity to shoot you. Once the infantry spot you or you get a couple shots off on them, they'll run toward you, which is a good way to lead them lower on the ramp and pick them off. When you get the majority of them, go ahead and run up and Lightning the tanks as fast as possible. There will be at least one guy directly behind you in the debris (usually), but there will always be a handful coming up the ramp as well. Pick the Brain Dude off with your Sniper Rifle and you're golden.
Stage 2 isn't too difficult, but it's long as hell. You need to be spamming Ravens Panic and using it to help you pinpoint enemies. It's early enough in the play through that you should be able to take out a handful of them before your spell runs out. Again, you can couple this with fire bullets if you need to. Take out Brain Dudes with your Sniper Rifle when you can and fire bullets when you can't. There is a Sniper on this level that you'll want to snipe as well. If you don't immediately run forward on this part, you can get a headshot off on him and kill 'im instantly. This is your first encounter with a giant, and you're going to want to hang back as far as you can but still maintain a decent distance enough to shoot with your Assault Rifle. You'll get the hang of this and eventually notice that these dudes have really weird pathing issues, and you'll be able to exploit this no problem.
On Stage 3, the Elemental Shotgun is your best friend. You'll want to take the guys on the stairs out with an Assault Rifle, and you'll probably find the dudes in the tunnel easier with this gun as well, but everyone else after this is almost always easier to take out with the Wind Blast, especially once you get to the Subway. When you get to the Airport, Ravens Panic and fire bullets are the best way to go, but be mindful of the Sniper. When you take most of the guys out in the middle, you can run at him and shoot him with your Assault Rifle. The closer you get, the more bullets he'll take, but even one of them will hang up his sniping animation. When you get aside and start clearing the way for the planes, take out the dudes in front of you, but hang back and snipe the guys on your left and right. If you don't move too far in either direction, you'll be able to get headshots on them without them coming for you. Then head to the right with fire bullets and take the Brain Dude out immediately. Same thing with the one on the left, and then in the middle. Boss strategy is the same one you use on lower difficulties, but you need to pay more attention to eyeballs coming at you, as they'll take you out a lot faster. Shoot them with an Assault Rifle when you need to, because it takes less MP to reload. The Chain Gun takes way too much MP and time, and can flub you up on your Lightning casts.
Stage 4 is pretty straightforward, and there's not a whole lot that's likely to get your goat, aside from the big ambush at the end. You CAN use Tornado here to help you out, but you need to run like hell in the opposite direction as soon as you cast it. Use Ravens Panic a lot, probably coupled with fire bullets, and just be patient during this part. Running around too much is likely to get you killed, and you only want to focus on individual pockets of enemies, instead of taking a bunch of guys out at one time. If, for some reason, the section with the railroad tracks does get you hung up, run from the Giant and start shooting the pockets of guys as you make progress through this part of the level. When you get toward the end of the railroad, hang back a bit and use the Giant "trick", which WILL work on this part. Then just go through it like you normally would, and be mindful of the explosives here. You can shoot them, but you want to make sure you have enough distance between you and the buildings. The explosions are big. Elemental Shotgun can help in the mine/tunnel.
Stage 5 is pretty tricky, but if you hang to the right as much as possible, you can avoid a lot of trouble. When you get to the tanks, you can avoid all of them except the one blocking your path to the forest, which you will need to use Lightning on. To avoid tanks, you need to spam Dodge and just keep moving in the direction you need to go. Cast Ravens Panic as soon as you see dudes so you can take 'em out, and spam it when you're avoiding the tanks, because there will be some dudes running around near them that you'll need to impede. Definitely use the Elemental Shotgun in the forest, and Dodge your way toward the Screamers (what I always called Spirit Spitters) and take them out as soon as possible. It's foggy, but you should be able to pinpoint their origins fairly easily. Don't worry too much about the infected dudes, but you can use your Shotgun's Wind Blasts to slow them down on your way to the Screamers. Once you take 'em out, focus on the infected dudes, and then you can either exploit the giant or just run around him. You can cast Tornado at the blockades, Ravens Panic and then hang back to shoot them, or even use the Elemental Shotgun to push forward, do whatever you think is easiest/safest.
Stage 6 is pretty much a repeat of Stage 2, but is a LOT tougher. The guys here take a lot more bullets to kill, so you'll probably want to be using the Ravens Panic/fire bullets combo as much as possible, but if you play your cards right, you won't have to all that much. There's a Sniper in the same place as Stage 2, and the same tactic works on him as well, though you need to shoot his face a couple more times. There are a few Snipers around the corner at one point, but because they're around the corner, you'll be able to easily ambush them. Don't even bother with the Sniper at the Yellow Brain Dude, because you should Snipe this Walnut Head to begin with. The ones you can't Snipe, shoot as fast as you can with fire bullets. For the ones that have dudes around them, use Ravens Panic to hold them up and take the Brain Dude ASAP, then you can hang back and take the infantry out. When you get to the tanks, use Meteor to take them out by casting it around a corner, but maintaining your cursor in the direction of the tank. Run away as soon as the casting animation stops and don't stop running until the buildings are done collapsing. Don't run forward until the meteors stop falling. There's only two tanks that you NEED to take out, as the last two you can actually Dodge past to trigger the cut scene that leads to the boss. Don't worry too much about losing your MP either, as there's plenty of guys to build it back up before the second tank. Just be mindful of the infantry in this area. The Giants in this level are plated and can take a long time to kill, and the very last one you have to take down is likely to shoot you, but if you hide behind the tank debris, you can usually get him to hang up like all the other Giants (this one, for whatever reason, has less issues with his pathing). Just use your Chain Gun on the boss, pretty much never stop shooting. When you get past the part with his shield, you can electrify your shots for extra damage (this isn't really necessary during the shield sequence). His weak spot is his back, so whenever he gets too close for comfort, just stop shooting and Dodge behind him a good distance and then you can resume your onslaught. The ghosts are taken out very easily with the Chain Gun as well, but try not to waste Elemental Shots on them if you can. Just make sure to Dodge his fireballs and whenever he has his hands up, shoot cars whenever you see them. You don't want them blowing up unexpectedly on you and ruining your progress. It only takes one Lightning strike to finish the fight when he starts slogging around, and it'll register as a hit even if he's moving around after you cast it (the important thing here is just initiating the spell animation).
This is fairly vague, I'm sure, but you should have AT LEAST 3 play throughs under your belt by this point, so enemy locations and the best paths through levels should already be worked out. Like I said, I never found a reason to change my tactics up too much during my last two play throughs, and I certainly didn't spend ten minutes shooting at tanks. The big thing here is your Ravens Panic and slowing down the way you play the level just a tad bit. I think I clocked a 26-minute finish on the last level this way, and I only died a handful of times. I never scored less than a B on any level regardless of difficulty.
Some tips that weren't mentioned above:
-You can cast Elemental Shot on a gun and then switch weapons to retain it. There's no time limit this way, and you'll have it reserved for the situations that you need it. The only thing to note is that cutscenes will reset this, but you can have 3 of your 4 guns charged at any time and even fill your MP back up in the mean time.
-Clicking Right-Stick and entering "Aiming Mode" will pull your gun out, as well as zooming in the camera and increasing your accuracy. This is extremely important for when you know there are going to be enemies around the corner, but Alicia has already stashed her gun away because you haven't used it in a while. People tend to just pull the Right-Trigger when they see enemies, but on the higher difficulties the delay in time between trigger pull and your first shot fired can be the difference between life and death. Preparation is key, I suppose.
-When your Shotgun is charged with Elemental Shot, you can cause enemies to stumble and even move debris at surprisingly long distances. This is beneficial for two reasons: Spamming your 16 shots gives you ample time to close the distance and take out enemies, and debris can actually be moved through walls, given that it's still within range of your shots. This is why I recommend the Shotgun most on Stage 3.
-The higher rank/Priority an enemy is, the more MP you'll receive upon killing them. This means that on the later Stages, you'll usually be able to cast more spells in a row than you think, provided you manage to nab a kill or two in the process. On Stage 6 especially, one infantry can usually give you enough mana for a Ravens Panic-Elemental Shot combo.
-The weak spot on the boss is actually his back. The game tends to skip the bit of dialogue that lets you know this. He's greatly susceptible to the Elemental Shot of your Chain Gun, as are his Snakes. Regardless of what weapon you use or what body part you're aiming at, electricity will arc from his body whenever you deal damage to him.
-Don't forget that you have instant cover at your disposal, thanks to your Ancient Wall spell. You can either hide behind it and wait for your mana to regen, or you can cast it and immediately start Dodging in the opposite direction if you know you're going to die. The latter method isn't always going to save you, but it can, and that's important.
That's about all I can think of that hasn't been mentioned in the other two guides, so good luck to anyone who doesn't have this one yet. Always remember though, don't adhere to a guide 100% if you find something that's not working for you. The beauty of video games is that they almost always allow you to experiment and develop your own technique for things, and Bullet Witch isn't an exception to this.
Cheers!