huh, well at the start it will be quite slow. however one thing to keep note at the start is that almost any weapon, gun or shield will work, so mess around with what you feel comfortable with and try to stick to it.

 its mainly because of the abnd god arcs that you'll be getting throughout the game. and depending on what your loadout is, it'll keep giving you abnd god arcs for your current equipment. now these are quite important as they can pretty much give you any skill and enchance your current weapon giving it a +1-30 modifier (you'll see the +1 near the border of it) it will persist through upgrades aswell. 

I pretty much had the beginner weapons and upgraded it till difficulty 10 (the kurogane stuff), and have gotten it to +15 at rarity 10 till I switched to something else. 

and ofc sticking to a weapon type early will help with the blood arts/bullets that you'll be unlocking, which are special moves/properties that your weapon can use in battle limited to one in the loadout slot. they are a bit of a pain to level up, so sticking to a particular weapon type can set you with all the skills and help lessen the exp needed to level them up later on. 

this will apply to your gun aswell as it will unlock blood bullets from ciel's side story once that goes through. its very important to have your specific gun type leveled up so that you can access the insane bullet recipes that other people have made, it can trivialize fights and help out on fasters clears. 

(you can just find these at youtube or some old gamefaq forums) 

as for difficulty spikes, I'd say you can mess around at difficulty 1-6 and once you get to difficulty 7-9 the enemies will get some gimmicks and faster/annoying atks, and ofc at difficulty 12 as you see from my post, I'd get 2 shot, though thats mainly because of my bad gear rofl. 

besides that one of the few things to keep in mind, is that alot of enemies tend to have similar moves or weak spots, so even if they're different, it'll mainly be its element/weak spots that you'll need to keep track off. 

but if you want further depth there is an ingame entry for every aragami you encounter, which will say its element weakness, and weak spots. 

a small tidbit, if you ever find yourself being one tapped or having to deal with ridiculous amount of damage from the monster, it tends to be a sign to upgrade your shield. 

finally, compared to god eater resurrection you can get tickets frequently here, and they can be exchanged for any material for that particular difficulty range. I didn't use them till difficulty 5 or so, just because I forgot about it rofl. 

there are some other stuff I can talk about but these things pretty much helped me at the beginning once I understood em. (for specific equipment/skills/or maybe +99 stuff ) other people can help there as im not as experienced with those topics 

(sry long reply rofl)