Heartbound Loop questions

How much armor would I need to lower the damage to 100 from 350?

How much does that number lower if I run AA?

If more that one procs at the same time do they add together or are they treated as separate hits?

Well, it "is" physical dmg so things which reduce it will work. Check wiki to see how much armour you will need:

http://pathofexile.gamepedia.com/Armour

Also, check this topic:

https://www.pathofexile.com/forum/view-thread/1400408

It probably will clear few of your concerns.

Cheers