Okay, so I was thinking something different. Thank you for clarifying that \o/
What I had in mind was to have the group prep an NM in appropriate level range. Let's say, newcomer is a lv8 in Anemos and the friend is a capped lv60 in Eureka:
- The group can pick any NM that syncs the helper friend within 8 level in difference vs the newcomer.
- Once the NM spawns, instead of killing off the NM, let the helper gather up mobs -around- the NMs instead while making sure the mobs aren't anywhere higher than 8 levels vs the newcomer; I find 4-6 higher levels are the sweet number. But sometimes even 1-2 or 7-8 higher levels in difference are still faster than any other cheese strats I've read due to sheer number of mobs you can pull and easily kill.
- Drop those mobs down to critical HP then hit that "Lv Sync" before finishing off the pull while making sure both helper and newcomer can see each other's name in party list lights up.
I'm not entirely sure how your DC's NM etiquette works. Over here if you're the prepper, the group have roughly 3 minutes before the NM is pulled provided the PT is being called, and additional 1-2 minute before the NM dies. This is plenty of time for a massive experience gain. No deleveling or exiting party required. One-two levels per NM is almost guaranteed; more if the helper is uberly geared with elemental bonus items unless the NM is pulled early for whatever reason. Bunny FATE is also available for the abuse if they're around & will not disrupt anybody in the zone.
The only downside to this is if you happened to zone in into juiced instance with no appropriate NMs left spawn. But between Anemos' least activeness and the other 3 zones providing bunny FATEs, I find that particular situation is rather... uncommon. There's always at least 1 FATE that can be used to abuse the lv sync.
EDIT: In other day I found somebody shouting in Pagos about leaving the lv25 NM alone to facilitate this abuse and the zone complies. They took their sweet 10 minutes before eventually decided they've had enough. While I think this is also viable, I'm not quite a fan of holding the NM on hostage, but I guess it also worked... not something I'd do though, hence my 3-5 minutes limitations.