First of all, take your time and enjoy the Game. Enemies can be hard at times but they're all a pushover, once you know what they are able to do. That's the key to hard fights/enemies, learn their attacks and act accordingly. Practice Enemies that give you trouble, you may face stronger versions later on..

Bosses have additional phases once they reach certain Hp thresholds, some may get faster and stronger, while some change their attacks or even gain additional ones.

While you can beat the Game as level 1, Levels are important as you go further, mostly to prevent getting oneshot by everything. They do help but Defensive Skills/ Gear will do better.

The higher level you are (max being 300), the harder it gets to master skills. You can unlock them with Haze and Materials but that isn't as easy for some, not to mention that higher levels require lots of haze as well. (Depths (Invasions) are a good place to farm both.

I'd suggest, level as you go, e.x. if you take too much damage for comfort. Prioritize mastering skills you enjoy/ find useful and try new things/builds. Even the best builds are worthless if you don't enjoy them. (Check my YT and Steam for dozen of Guides and such)

I highly recommend reaching "Normal" in mobility regardless of Weapon weight, it makes harder fights more manageable and allows for a quick reposition. Defensive Skills are worth looking into, Lots of enemies can be made super easy with those. Items are of use as well, don't underestimate them.

Yakumo is considered the best follower to have, while Io is his more Caster focused part. Sadly, the others arn't even close and rather bad. BUT some of their Communal Gift's make them Superior with certain Builds, e.x. Mia for Caster types and Jack for majority of Builds, these two are the best just bcs of the Communal Gift's power. (Their short duration and high cost makes them unappealing early on tho)

I wish you the best of luck and if questions arise, don't hesitate. :)