How do i build and play as Rhaast?

For some reason I can't get a double kill with Rhaast, which is ironic because as SA Kayn, I got a double kill no problem (which lead to my killing spree)

I know Rhaast heals off of some spell damage dealt, so should I go Death's Dance with some armor or something?

Any suggestions will be helpful. Also if you guys could tell me what masteries? :D