I would try to stick to leves as they give good xp and provide a nice steady income if you're hard up for cash. Select 1 star below ur lv as they are easier to kill and thus makes clearing leves faster. Leves will on average give 250-300xp per mob kill with a reward of 3k+ for 5-10 mins of work.
You probably won't see any parties below lv 30 til the server merge repopulates the servers. If you grind try to stick to mobs same lv or just 1 above. Mob difficulty vs time investment on anything higher isn't worth the time.
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