Okay, for grinding at least. The best way I can see is to rush normal until you clear the campaign and just farm the final map. You just have to fight the boss and if you kill it then you win (don't have to kill everyone). If SSS rank, you can net 8k-9k EXP per run. If S, 6-7k. Let your buddy be meatshield and stack her with EXP-UP Material. She won't be much use.
Use Shotgun, nuff said. That thing is ridiculous and I'm starting to suspect that the damage calculation for the spreadshot is wonky and each bullet does the damage that was stated (so if it said 350 dmg, it's 350 x (number of shots, say 10) = 3500 damages per close range shot). If you have the Material to reduce RoF, this is where it comes in because you'll shred through the boss pretty quickly.
Machinegun works too but try to aim for the dragon's weakpoint (chin). Its spread-shot is actually a good thing this time because a few lucky shots and the dragon goes down. If you suck at aiming, spray and pray is another way to do it.
If your aim is good (without using focus since that one auto-aim), use pistol/rifes to deal with the mobs as those have great aim. Sniper is busted because once you aim, it's hard for you to get out of auto-aim and the enemy will be near you before you know it. Sniper is broken (bad) and is only meant as a knockback/lucky-damage-if-hit-weak-point machine. If you want pure knockback, bring catgirl and let her spam Magicite, enemy will be continuously knocked around.
Special effects can be really useful in this game at higher difficulties. Lighting, Fire, and especially ice. With machine gun + Ice effect, you can stop enemies in track pretty easily. Fire provides DoT but also mini-stun which is also helpful in keeping them away.
I recommend just grinding the Catgirl's extra mission in ECSTASY for material. You'll have easiest time managing things and aiming won't be too much of a problem. Stack the catgirl herself for bonus item drop.
If you want glasscannon but easy time, focus on leveling the mage. The amount of mob she can lock-on can make short work of almost all maps. If you can stack her with elemental of each (Ice, lighting, fire), her passive (increase the chance of status ailment) can cause the enemy to be stunlock for ages. DoT also help in taking them down faster at those ECSTASY level. Beware that against Gunorc, she's really susceptible to their guns. A few shots in and she'll go down unless she's geared up (or you spec her to reduce gun impact.
My strat is to use Catgirl + Magegirl to pretty much stunlock them forever (Catgirl magicite has 100% chance of inflicting Lighting....or you can change to fire/ice too) so combine that with your attack and the mob won't be moving anytime soon. Just carry around shotgun for those spawnpoint and/or RPG for airborne bullcrap (helicopter).
ECSTASY (and Hard for many later maps) are NOT designed to be cleared by low level girls (20-ish) unless you have godlike aiming power and focuses on their weakpoint 24/7. Of course, earlier ECSTASY level can be cleared with lvl 10-30 but it takes time. Once you start running into those GUN-ORCS, that's where the nightmare starts.
GUN-ORC can stun-gun you to death in 1 round if you can't react fast enough and jump away....even then, another GUN-ORC can be shooting at the location you're recovering which puts you in quite a pinch. I'd recommend not going against them until you're in your 35-50 which is when you start allocating some of your skill points into defense and weapons are strong enough to actually poke at them safely (they still won't die in 1 shot but it's a lot more doable). They are very annoying, but that's probably the most fun I had with this game. GUN-ORCS require you to really know your surroundings because if they're hitting you from all sides, you're kinda fucked. It stops me from just charging in and actually makes me look around before engaging.
Overall, the game could really use a "disable auto-aim" feature because by the fucking god....that thing pretty much makes you unable to aim their weakpoint since auto-aim won't aimt their weakpoint, but their mid-point.
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As for Fruitsmootie's request....sorry, I can't get sniper to work too. I tried tuning the sensitivities but it doesn't help against the auto-aim feature. There really needs to be a way to close it in order to fully utilize sniper.
However, I can help elaborate on other weapons. Bow suffers the same issue as sniper but got off better due to its ability to actually do spreadshot (somehow) once you get its talents. Otherwise, it's mainly to look cool and for continuous short-range sniping. Bow is pretty much a no-scope close-range sniper.
As for Magicite and Grenade, they are situational utilities rather than damage dealers. Grenade (not grenade launcher, that one not as effective) is pretty much an anti-tank. It is similar to that FIM bazooka but a lot more spammable. But since you rarely face tanks in this game, it's quite obselete. Of course, if you have to deal with helicopter then it's bazooka (preferably FIM). Grenade also works to an extent as an AoE anti-personnel but you're better off using guns and spray to apply elemental debuffs.
Fire/Ice (+Lightning if you use Catgirl's personal weapon) Magicite has only 1 goal, to apply AoE Elemental debuff. Elemental debuff (burn, freeze, shocked) are not 100% chance. So it's not stunlock the moment you throw it. Still, throw enough and you'll start seeing them stack up. It can give you that breathing room for both you and your buddy to do something else. As it has minimum-range limit, you'll have some trouble throwing against melee enemies although it's not impossible. However, they are really useful against those big cluster of range-mobs (e.g. slimes, GUNORCS, etc.) because you throw one like spamming bazookas and give yourself a breathing room to do other stuff. Knockback is also a plus.
Reload/RoF mods are preferences but you can live without them. For me, I used RoF mods with Shotgun girl and uses an Unload and Run method. WIth faster RoF, I can unload all my seeds faster in order to get out of situation faster (e.g. rush in to kill something)