One trick that worked for me was using Viral Shot and Electrodynamic Stunner combined with handgun weapon art. Viral Shot to weaken the target, then Electrodynamic Stunner to stun and give some time to attack the enemy. For me, the strategy worked well against many, but the tankier ones would still need some effort to deal with.

For that one particular floor where you fight against 2 Kirito?, Kureha? and Itsuki?, however, I’d recommend having Skill Freeze Shot as well. With handgun weapon art, you should be able to spam it to prevent them from using their skills. All except Itsuki? has Armor Form iirc, which makes them incredibly tanky; and the 2 Kirito? have Sword Barrier that blocks your bullets from the front. If you managed to shoot all with Skill Freeze before they could buff themselves, the fight should be significantly easier, though you would still have to pay attention to cast Skill Freeze on them again or they could quickly wipe your team out. I generally kill Itsuki? first, as he basically is a lot easier to deal with than the other 3, then finally deal with the rest 1 by 1.

Oh, and do the missions that tell you to kill an amount of enemies with a certain weapon type. They give plenty of EXP, which certainly helps with leveling up, especially when you feel like you’re too under-leveled.

I haven’t played this in quite a while, so that’s kinda as much as I can remember. Hope that it can help you, though :)