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Paineel guards can be done at 20 with no faction hit.

Rivervale/misty thicket can be done around 30 or so, however you will experience resists and they're fairly camped. There are lvl 40 guards inside Rivervale so watch out plus it can be kind of camped.

Kelethin guards can be started at 30 but again, there are some that are 40. Guards don't path off the edge so that's nice too - no need to worry about the lower newb guards jumping in and ending your shit.

Felwithe guards can be done 40+ but you need to keep them feared otherwise they will loh. Their drops are arguably the nicest.

Halas guards apparently can be done but the shape of the city is very straightforward and unfortunately you path right by the shaman guild with a GM outside. Halas guards are also on shaman guild faction so you will likely agro. Zone also requires swimming but since you'll have to be 40+ you'll likely have Dmf.

Kaladim guards are great if you can split them. They are all 40+ and I did them extensively on my shaman. Great cash camp if you bind at the Druid ring.

If you're truly evil you can do neriak guards. There are a lot of them and they are a healthy mix of 30's and 40's.

Efp and wfp are all level 40 guards except for a few here and there. Lots of them in efp to be had. I believe wfp raises faction with NFP so they are usually camped more often.

Qeynos is too varied. I would just avoid, however, you can do nk/wk guards which could be alright. Never done em personally.

Erudin looked promising too but the fact that they're paladins means loh and likely blind/root. They are most L30.

Hhk guards are between 27-35 and are good till mid 40's. Amazing zem there though so that's worth mentioning.

Ak anon has roamers outside (two watchmen I believe) who patrol the path outside the city. Not too sure about inside the city but IMO there's tons of wandering reds who will likely ruin your day. I would advise against unless you find a relatively decent safe spot.

All in all there are lots of options. The general takeaway is that most guards spread across the world are either early 30's or early 40's. I wouldn't start any of them until they were a healthy shade of dark blue. Some are casters but you're a necro and should be able to handle that.

Good luck.