I feel your pain.  The real problem is that to be competitive in high arena you need [Arbiter / Lysandra] + [Some super high DPS with ignore defense] + [some buff stripper and/or damage blocker and / or Lillitu or Siphi]

There used to be more room for creative combos with crowd control or turn-meter manipulation but now you hardly even get to engage those before they are wiped out or overpowered.   

I have some of those key pieces so I've been OK - but I'm often the only person on the page without Arbiter as a lead for multiple pages (500 badges to go.....). 

It would be nice if they'd cap Speed and put a 3 turn no-rez debuff on people rezzed in Arena but I don't think they're willing to go that far.

What role is Khoronar filling on your arena team?

I  don't  use  Khoronar  in  arena  because  his  damage  is  extremely  weak  and  he  is  either  dead  when  i  get  a  turn  or  the  enemy  team  has  block  debuffs  so  his  provoke  does  nothing  other  than  gives  himself  defense  up  which  doesnt  help  the  team.  My  hero  selection  is  mountain  king  in  shield  set,  iron  brago,  rhazin,  brogni,  doompriest,  skullcrown,  seeker,  fahrakin,  sepulcher,  scyl,  apothecary.  My  seeker  with  270  speed  and  skull  crown  as  lead  isnt  enough  to  beat  anyone  on  the  arena  page  so  i  always  have  to  go  second.  Because  i  am  facing  arbitor  or  deacon,  they  usually  go  twice  and  their  nukers  near  twice  before  i  get  to  take  a  turn.  If  i  can  survive,  Rhazin  usually  resets  turn  meter  and  mountain  king  can  kill  the  speed  lead  unless  they  have  madame  serris  or  stag  knight  which  decrease  my  attack  so  then  arbiter  doesnt  die  and  i  lose.  Even  if  i  still  take  out  arbiter,  their  nukers  and  second  speed  person/support  take  the  rest  of  my  team  out  and  then  its  mountain king  vs  2  or  3  and  i  lose.  However,  if  the  enemy  has  any  combo  of  arbiter,  siphi,  krisk,  nekhret,  trunda,  zargala,  mashalled,  lyssandra,  tormin,  serris,  deacon,  duchess,  astralon,  candraphon  (which  are  9/10  arena  teams  on  the  page)  then  i  cant  even  touch  them  and  i  get  completely  wiped.  I  obviously dont  face  them.  Its  stupid  that  just  because  you  dont  have  certain  champions,  aka  arbiter/nekhre/duchess/siphi/krisk,  you  cant  compete.  Obviously  i  will  get  faster  gear  over  time  but  as  i  improve,  so  will  all  the  other  accounts.  Looking  forward  to  trying  brogni,  khoronar,  mountain  king,  and  doompriest  all  in  shield  set  and  hopefully  their  nukers  die  from  reflect  damage  but  i  am  long  ways  away  from  that  many  good  shield  sets  and  that  will  only  work  if  they  dont  have  buff  strip.

Khoronar is an excellent champ, including for arena, so I think if you're not using him there, that's a mistake. Though of course, against enemies with block debuffs, he'll be a lot less useful if you don't have a Serris of your own to strip that first. If you're facing Serris and you know you'll be going second, you'd have to ask yourself if the shield set is really worth giving her the chance to put block debuffs on her own team and fear on yours - then again, if you don't have the shield set, she might use Midnight Ritual first instead, so it's kind of pick your poison. 

But yeah, you'll need more speed - maybe not so much as to go first, but enough that they don't take two turns before you at least (unless you're facing Lyssandra, then it might be inevitable). 

Also, you can go ahead and unlock your level 10 Great Hall bonus, all the GH missions are retroactive and will auto-complete if you've already unlocked the required level by the time you start the mission. 

To be honest, it sounds like you are just missing speed.  You have Skullcrown for Nuke + speed aura and Seeker for turn meter boost and increased attack.  I'm sure you have Warmaiden for AOE decrease def which you could deck out with stun set and mass accuracy, and then just speed tune your team accordingly.  You could even throw in apothecary as the fourth for further turn boost if. 

Edit:  I guess what I'm saying here is that the issue isn't a lack of champions, but rather an issue with the speed war.  I do actually think that the nature of the speed war with Arena is an issue, as it does dramatically take a lot away from the variety of champions that we have

RAID ARENA IS THE WORST I HAVE SEEN IN THIRTY PLUS YEARS OF ONLINE GAMING !!