I know it's great having PSU back in some capacity!
I started playing just over a week ago during the Protectors event mission so levelling was good there, now at the mercy of normal missions.
I found that melee/hunter classes are so difficult for soloing at lower levels. I tried lots on Neudaiz and Parum and it was quite tough.
Hopefully somebody whose been on clementine longer can offer more solid advice but I'd suggest being a ranger, stick with shotguns, launchers (b rank koltova) and twin handguns and you could probably survive by strafing, stuff like Plains Overlord on the highest you can probably run.
Seems like guns and tech are what I see most people using to muller content. I play protranser as I used to love it and find I barely ever pull melee out now :(
Running in groups is easiest though having somebody else for mobs to target and there are always people at Plains overlord, military subway that I've seen. Suggest joining random parties is probably best way to level without bankrupting yourself!
Hope you find some semblance of help in that Good luck!