I know I've been mighty negative about Eureka overall, but I figured I'd tell others what I've been doing and what seems to be the most "successful" way to play overall in terms of currency grinding, level grinding, and lootbox grinding.
This only works on an active map with lots going on. If you're on a dead map leave and try to get another one.
First of all, it behooves you to find a full 8man. Then, you go to one of several places depending on your level where the mobs are significantly higher than you. High enough to make the XP worth, but not so high that you can't chain. At level 7 I think I was on leshys for example. Some mobs are SIGNIFICANTLY less dangerous than others at the same level, btw. The tank and/or a ranged will single pull mobs back to the group, which is in a safe spot with nothing spawning on them. Everyone just burns the mob down continuing to chain.
At the same time, people on the map should HOPEFULLY be spawning NMs. NM spawning by itself can be really bad for XP especially if you are higher level - if you spawn lower level NMs you'll be getting 1xp per kill and you wont get chain credit iirc. So basically the gist is to be grinding yourself and hope that someone more charitable is spawning NMs. When the NMs spawn have your 8man drop everything and kill the NM. Then go back to where you were farming before until the next NM spawn.
This will be soul crushingly unfun overall but it does seem to be the most efficient way to play the map. Basically if you're not in a big party training high level mobs to your group to burn down efficiently while waiting for NMs you are wasting your time.
Unless you want to just afk in spawn, do something else, and wait for NMs ofc which will get you less overall rewards in game but if you've got monhun up on your PS4 or something next to you I'd argue that's equally efficient, just in a different way
(the fact that I even have to ponder being semi-afk really speaks to the quality of Eureka's content, sadly)