I literally just started Eureka 3 days ago. I feel that Anemos is still doable to catch up on. I choose to solo until Lv7 then I joined NM train to hit Lv10. Do the challenge log (either in party or solo). It really helps you level quicker.

I can still see people doing trains in Primal. Perhaps depend on how long the instance has been going, sometimes they're active and sometimes not so much. Just shout for an invite. Unfortunately, you may not be able to do much if the train is spawning the high level NMs. I had been leeching EXP, help raising, and dying from the boss' area wide attacks. I look forward to level more so that I won't get one shot.

Hugging the wall is what helps me get past the high level mobs solo. If you find them clustered together, try to observe and look for huge gap between them to sneak past.
But if you died on your way to your party members, just shout with <pos>. Very often people will come to your rescue.

For Pagos however, judging from a lot of unpleasant feedback I see in the forum I feel it's a content where I should catch up on as soon as possible or sit out and wait for hotfix/changes/nerf/etc. Most likely I will go for the latter.