I managed to upgrade my barbarian slightly recently, which is allowing me to handle Monster Power 5 Act 1 just fine. Stat-wise, he has (unbuffed) 38.8k health, 640 all resistances, 6.5k armor, 1.1k life on hit and is around 113k DPS. I use a Scoundrel as my companion for the increased critical hit chance. Unless I get supremely unlucky or play sloppy, I have very little problems staying alive for the most part.
Currently, the way I farm Act 1 involves areas which is well documented as mob dense zones. I start by hitting the Cemetery of the Foresaken and backtracking to The Weeping Hollow. Depending on your luck (or lack thereof), you can find anywhere between 2-4 elite packs and possibly a treasure goblin. It’s up to you whether or not you want to handle a treasure goblin prior to having 5 Nephlam Valor stacks but more than likely he’ll teleport out unless you have the damage output.
Once I clear most of The Weeping Hollow, I port over to the Festering Woods. Here, you can find anywhere between 3-5 more elites and/or an event as well as an occasional treasure goblin. If you can you should probably hit the two underground zones to get the guaranteed elite. By the time you finish, you should have your 5 Nephlam Valor stacks.
For the next part, head over to the Fields of Misery. With your 5 Nephlam Valor stacks you can hunt down the Keywarden. I tend to try and clear most of the zone. The dungeons are optional as I find the meat of the area to be above ground.
From here, you have the option to restart the game or do a Whimseyshire run. Why Whimseyshire? Outside of having several elites and champion packs, you also have the slim chance of finding a unique that may drop a Spectrum or Horaldric Hamburger. If those are well rolled, you can make a good hundred million gold or more depending on the damage output.
Regardless, this farming route focuses on a few goals. One is to kill numerous white mobs at a reasonable Monster Power and thereby receiving some decent experience for your paragon leveling. Next, you want to get your Nephlam Valor stacks to 5 for both Key farming, better drops and the experience boost. Next, as stated just before, you can farm for your Key (I’ve been lucky each time and it dropped every single time). Finally, beyond killing elites in the hope of finding legendaries, doing these runs on the higher Monster Power will, indeed, help boost the rate of finding legendaries not just on elites and champion packs, but on white mobs as well. On one run, I found three legendaries as well as the key.
Also, for a barbarian most of these mobs are pretty safe and easy. The biggest difficulties I’ve encountered thus far have been getting an unlucky mix of affixes on elites and champion packs that wiped me out. But the white mobs themselves are not dangerous for the most part and only if you play sloppy will you see any danger. I’m considering bumping up the Monster Power to see how I can fare at the next level. But so far this route has proven quite positive and yielding a few decent legendaries as well as boosting my experience fairly quickly.