Is there any way to see how after a game how the AI did a build. I am curious because I have been playing single player on nightmare to try some different builds of my own and there are several things that I would like to do but cant really figure out.
No, there isn't, unfortunately.
1) In one game I was standing next to my crystal and fighting the Unclean Beast (both of us at level 20 I was oak). I had about 1/2 health and it stayed there even with crystal. I couldnt even take him down in health at all and I was hacking away for about 1min. I have actually experienced this 3-4 times up against other characters.
Probably a combo of hps and life steal, possibly provided by artifacts.
2) I just played a match with unclean beast on my team. At end of match I had obtained 56,000 gold killed about 25 demigods and 250 grunts and only died once. The beast had around 82,000 with only 5 demigods killed and 74 grunts killed. How did he have so much more gold than me?
The nightmare AI gets buffs that cause them to obtain gold and XP at ridiculous rates. Play a single player tournament on easy and you'll have an idea of the buffs they get.