Would wanna ask the same question too.. I had choices to either awaken Kris or Wukong .. Any advice .. ?

DD nightmare

a) A.Ruri,A.Feng Yan,Diao-chan (supports,debuffers) aka the holy trinity

b) Strong backline dps like A.Chancellor,Klahan,Lubu,etc.(hero with strong aoe pierce and buff reduce is preferred)

c) Last slot hero depends on which DD of the day ie Yeonhee for reflect enemies,etc.

World Boss

a) Just inspect the top 10 team (inside the WB page,the visit button next to the player's name) for team composition and how to gear them properly.Boss guide (which skills to use/counter) can be searched on Youtube.

Arena

a)Your current Arena team is extremely outdated,inspect top 30 teams in Arena to get the latest meta teams composition.A.Kris should be the better option,he fits in any pvp team currently.

Suggestions

Already say this quite a few times (in Reddit mostly) so going into emotionless parrot mode.

For new/intermediate/returned players,don't sink your resources (rubies,topaz,awakening stones,etc) into PVP yet,Arena meta changes every time new special heroes are released/awakened.You are not going to catch up reasonably if you invest heavily into PVP and neglect your PVE farming capability.

Focus on building/rebuilding your PVE teams for DD,CR,WB,etc.Once you have resources farming settled then only move on to improve your PVP team.

HOWEVER,my suggestions only apply to players looking to play the long term game,IF you are casual playing on and off,then you are free to do whatever you desire.
PS:Try to separate your question into short paragraph if long wall of text,easier to read.
Refer post above for Arena.

i just pressed another 40 times, lethal dmg jewel
i dunno seems like increased rate to me. previous event i pressed 5k ruby and got 0.
For Nightmare Daily, Diaochan is a must, generally i use Klahan as backline, Fengyan, Diaochan, Ruri. Last slot Sieg, Yeonhee, Dellons, Mao Song (for zombie day). It depends one the day.

Example for today's Fire Ele, I used Klahan backline (main dmg), Ruri (second dmg), Feng Yan (his awaken and cd reduce), Sieg (cd reduce), Diaochan (void shield buff). Today's daily is one of the harder ones because Lubu destroys shield easily and chancellor cooldown spell is annoying.
Hard decision . I think Awk Kris. It comes down to your heroes i guess.

My Aris is still lv30. 

My Ingrid also stuck at lv38. I want to fully LB her..... Keep losing at arena at stuck at 3600+.

Woot? last month not having the hidden master dungeon and they  give 8 HM selectors for free?
Don't tell me you didn pick Roro ?