DH + Rathma's Necro 2-man team advice

A friend of mine came back to D3 and decided to play necro. He started with Rathma's (obviously, since this was haedrigs gift) and even though he hasn't tried any other builds he is not willing to change. He is not very experienced with D3 in general so pushing even for the low end of the LBs isn't likely at this time. We just want to have some fun and test our limits.

I 'm playing DH this season for solo pushing and I 'd like to use DH when i party up with my friend.

I have been playing solo only for a very long time (last time i did serious group GRs was back when Tiki WD was all the rage) so I m at a loss when it comes to class synergies.

So far I have been playing a zdps DH that seemed to work fine, providing enough toughness for him to survive, but the dmg bonus seemed a little low.

This is my build... sort of. The stats are a bit off, because I cant log in the game now and change gear to import my zdps items, so I 'm going from memory.

Questions:

1. Should I go for the stun on vault: rattling roll to proc Krisbyn's for the necro? or does this build up the CC resist too fast since its stun + knockback

2. Is vengeance:from the shadows worth it for krysbin's proc with the 3 second freeze?

3. Should I swap vengeance for marked for death? Useful runes can be valley of death rune (AoE dmg debuff) or Death toll (healing)

4. Should I use Ess of johan amulet or bolas for grouping mobs up or is this kind of grouping useless for Rathma necro? I m reluctant to use this because it build up CC resist fast and results in lower Krysbin's uptime. Thoughts?

Also what variation of Rathma should my friend run?

For now he has this skill build with Reilena's Shadowhook in cube and Dayntee's Binding equiped that he uses for solo play.

Should he change it for group play or leave it as it is?

What is his biggest threat? Elites or trash mobs? Maybe I should just let go of the idea of zdps DH and just go Impale or UE grenades and kill stuff along with him?

Any help would be greatly appreciated :D

TL DR: I am DH. Friend is Rathma necro. We are not going for LBs, just for fun and testing our limits. What builds should i use when I play with him? What Rathma variation should my friend use?