Ranger solo

[Jimjam]

Pretty much anything blue you should be able to grind down safely as long as you have a reasonably good weapon.

Things that drop crushbone belts or legionnaire pads will be good. If in crushbone you can keep killing emissaries and the task master solo, or do the throne room with your shaman friend. Once the xp really slows down is the best time to hand in your belts/pads. Use kelethin bank to save them all up.

Oasis is pretty easy xp and can take you to 22.

Kurns tower isn't a bad option (if you are crazy enough you can save up all the bone chips to hand kaladim, it takes about 1000 bone chips to do a level in the 20s, iirc).

Highpass is alright, save orc/gnoll scalps to hand in for xp once the area greens out. The PGT can be a fun off-hand weapon to use for the proc until duel wield/double attack actually become viable enough to put a dps weapon in your off hand. There is also a big Str pad shield that drops there and an invulnerable aura potion (ECmarts for circa 700pp).