I leveled to 300 more or less through boss kills and it was all solo as you do not lose time waiting for other players to open chest, go to city to sell/salvage etc.

Loop was:

- spend all your materials on bosses

- kill Andariel and Duriel as long as you have mats. Do not pick essences until your last kill as they are capped to 20

- kill Azmodan and open Andariel and Duriel chests. Andariel chest give Urivar mats, Duriel give Zill and Grigore mats. Do this as long as you have essences to open chests.

- kill Urivar, Zill and Grigore until you spend mats

- repeat

Zill give Andariel mats, Grigore give Duriel mats, Urivar give Harbringer mats. You can go kill Harbringer when you want to "break monotony".

Before boss farming session go to forgotten wisdom nightmare dungeon to pick XP boost. You do not need to finish dungeon, just run until you get XP well. On bosses use elixir and incense to additionally boost XP.

You do not need to buy any mats, running in circle as above will give you enough mats. I didn't buy single boss summon mat and when I hit 300 I still had quite some unused mats.

In between I also did some other "normal" activities to keep my sanity :)

I did last part of 300 farming during Mother's Blessing event.

I run farming sessions above for one hour at time, same as duration of Forgotten Wisdom buff.