Do you have some tips and tricks for us?
Unlock those crystals where you can. Once you can hero run basically every turn, which you do want to do and also always leave yourself in a position to be able to do, it becomes somewhat easier. Jumping during a hero run gives you a tiny bit more time and might allow you to dodge things, however it can make other things a tiny bit harder.
During said charge/run times it can be harder to lock onto enemies so memorise their patterns if you do have to swap.
You can move the guns themselves on the customisation page. I did not realise this until long after I was struggling to get the parts I wanted on certain weapons.
Remember to take a few levels in grenade. I know they are annoying for normal enemies and you can dispatch them quicker with the other two types (especially if you are dual wielding) but if you leave grenade to lag then you will start to take ages to level the other two and a level is a level as far as stats are concerned so yeah. Grenades can also help a fair bit with special enemies if you have similarly special grenades. Also don't be afraid to figure out a special enemy's weakness and start a battle again if it is a hard one. Medic kit wise other than when you are being poisoned in a dungeon then you can leave it off for most battles.
The autofire accessories are nice, however if you can learn to do without them (maybe leave them off one character at first, then two) then the additional slot is surprisingly useful.
Once the colosseum gets opened up properly it is well worth doing.
Fabrication can be gamed slightly and is well worth doing rather than just trying to buy things.
As far as customising weapons goes a jack of all trades build is not great. Focus a bit more on a certain aspect but do not entirely neglect the others.
Learn where certain drops are.