This is my solo crypt run.
0.00-0.40: gear showcase
0.40-2.00: gathering monsters before crypt
2.00-4.20: low level crypt run to spawn enemies
4.20-6.50: quest+chest for 2 stacks and level 2 clear
6.50-14.45: level 1 clear
14.45-15.30: clear enemies in front of crypt + teleport
First I equip high ehp gear with lots of life on hit. I run around fields of misery to find the crypt and while doing that, I use locust swarm-pestilence to create a trail of monsters without killing them. Once I feel I have gathered enough monster, I enter the crypt.
I run through the crypt aggro-ing every monster. The trick is to let the mothers spawn extra zombies for more xp. For a witch doctor, this is the hard part, because he cannot defend himself while prepping. The life on hit ticks on the locusts is what keeps him alive.
Get to level 2 of the crypt, switch to your high dps gear and open the chest and do the event for 2 quick stacks. Clear level 2 while you're there and run back through level 1, killing everything. Skip hard elites or do them for more stacks. Outside, the trail of monsters is still waiting for you for some quick xp.
In this run I got around 35m xp in 15 minutes. I recommend doing this run with a full party, since 3 others can prep elites in festering woods, so you can clear the crypt with 5 stacks in the same amount of time plus 30% party bonus.