Billy Dee will fly to Eider and Help Shmitty with his next attack. 

Shmitty will Hide

(Read below if your pp beats my Stealth 13)

I have no idea how to do spoilers on my phone

Shmitty moves to M15 and envelops Eider with his Hunter's Mark 

Str Check for Water Elemental Attack 6

Acrobatics to escape grapple 15

Disengage to fly up 15 ft in the air and over to A8

Sentinal AoO Claws Attack: 21 Damage: 5

(Eider can't see you so..)

Eider moves another 30 feet to the center of the arena, lays down prone, and readies his spell yet again.
"I've seen you fight many times, laying targets are harder to hit right?" He says to the empty arena.

(Attacking a prone target gives advantage to melee and disadvantage to range.)

Complex:

Eldin waits patiently.

The fog is 10 feet away. Gewonnt is out of time.

Gewonnt hastily makes his final preparations. (Do you wish to make a move?)

Arena 7: Complex

  A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P
16 N.  .  .  .  .  .  .  =  .  ._ ._ ._ ._ ._ ._
 ===%.....%=============================%.....%==
15 E€  ,  ,  ,  -  -_ -_ =_ -_ - _-_ ,_ ,_ ,_ ¤_
 ===#..=##=======%%%%%=======%%%%%=======##=##===
14  ÷  ÷  ÷  ÷  =_ ._ ._ ._ ._ ._ =_ ._ ._ ._ ._
 =..÷........÷..=.................=..÷÷÷÷÷÷÷÷÷÷..
13  .  .  .  ._ %_ ._ ._ ._ ._ ._ %_ ÷_ ÷_ ÷_ ÷_
 =======%%====%%=.................=..÷÷÷÷÷÷÷÷÷÷..
12_ ÷_ ._ =_ ._ =_ ._ ._ ._ ._ ._ =_ ._ ._ ._ ._
 =÷÷......=.....=======%%%%%=============%%======
11_ ._ ÷_ =_ ._ %_ ._ ._ ._ ._ ._ %_ =_ ¬_ ._ ¬_
 ====%%====%%====..=,,,,,,,,,,,=..=..=¬¬¬.....¬¬¬
10_ ÷_ ._ =_ ._ %_ ._ ,_ ,_ ,_ ._ =_ %_ ._ ._ ¬_
 =÷÷...___=_____=...,,,,,,,,,,,...=..=¬¬¬.....¬¬¬
09_ ÷_ __ __ __ =_ ._ ,_ ,_ ,_ ._ =_ =_ ¬_ ._ ¬_
 =÷÷..._________=...,,,,,€,,,,,...=..=======%%===
08_ ÷_ __ __ __ =_ ._ ,_ ,_ ,_ ._ =_ =_ ¬_ ._ ¬_
 =÷÷...___=_____=...,,,,,,,,,,,...=..=¬¬¬.....¬¬¬
07_ ÷_ ._ =_ ._ %_ ._ ,_ ,_ ,_ ._ =_ %_ ._ ._ ¬_
 ====%%====%%====..=,,,,,,,,,,,=..=..=¬¬¬.....¬¬¬
06_ ._ ÷_ =_ ._ %_ ._ ._ ._ ._ ._ %_ =_ ¬_ ._ ¬_
 =÷÷......=.....=======%%%%%=============%%======
05_ ÷_ ._ =_ ._ =_ ._ ._ ._ ._ ._ =_ ._ ._ ._ ._
 =======%%====%%=.................=..÷÷÷÷÷÷÷÷÷÷..
04_ ._ ._ ._ ._ %_ ._ ._ ._ ._ ._ %_ ÷_ ÷_ ÷_ ÷_
 =..÷........÷..=.................=..÷÷÷÷÷÷÷÷÷÷..
03_ ÷_ ÷_ ÷_ ÷_ =_ ._ ._ ._ ._ ._ =_ ._ ._ ._ ._
 ====##=##=======%%%%=======%%%%%=======##=##===
02_ €_ ,_ ,_ ,_ -_ -_ -_ =_ -_ -_ -_ ,_ ,_ ,_ ¤_
 ===%.....%=============================%.....%==
01_ ._ ._ ._ ._ ._ ._ ._ =_ ._ ._ ._ ._ ._ ._ ._
   A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P

Key: '=' High wall (8ft.), '#' Window, '÷' Furniture, '_' Stage (2 ft.), '¤' Plant, '€' Tree, '¬' Counter top (4 ft.), '%' Door, '.' Marble floor, ',' Grass, 'A-Z' Contestants

Yes.

Eldin's eyes widen as he realizes that he had made an oversight in his planning. He takes a look around to try and spot Gewonnt

Perception 6

The rest of the turn will depend on what he sees

Bog:

Eider walks out into the arena.

Shmitty reveals himself to cast his spell then disappears into the bushes (unless you want to hide then reveal yourself as you stated) (Eider may react)

Eider drops to his belly on the bridge.

Polly is sucked into the elemental. He slips free and moves to escape without leaving an opening, but Grianne is a huntress and won't let him escape, she claws Polly in spite of his efforts.

The mist begins to close around the sides, engulfing the pair.

([25]Grianne. Eider may React. [5]Shmitty)

Arena 7: Bog

  A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P
16  .  .  .  .  .  .  .  .  .  .  .  .  . ·.  .
 ===%.....%================================%..===
15  :  :  ~  ~  ~  ~  ,  ,  ,  ,  ,  ,  #  ,  ,
 =,,#:::::----------:::::::::::,,,,,,#,,#,,,,,€,,
14  #  :  -  -  -  -  :  :  :  :  ,  #  ,  ,  # 
 =,,€,,,::----------::#,,,,,:::::::,,####,,#,,#,,
13  ,  ,  ~  ~  ~  ~  #  ,  ,  :  :  #  ,  #  ,
 =,,,,,~~~~~~~~~~~~,,,€###,,:::::::,,,,,####,,,,,
12  ,  ~  ~  ~  ~  ,  ,  :  :  :  ,  ,  ,  #  ,
 =##€,,~~~~~~~~~~,,,,,:::::::#########,,,,,€,,,,,
11  ,  ~  ~  ~  ~  ,* :* :* :* ,  ,  ,  ,  #  ,
 =,,###~~~~~~~~~~~~~~~-------,,####,,,,,,,,######
10  ,  ~  ~  ~  ~* ~  -  -  -  #* ,  ,  #  ,  ,
 =,,,,,~~~~~~~~~~~~~~~-------~~~~~~~~~,,#######,,
09  ,  ~  ~  ~  ~* ~  -  -  -  ~* ~  ~  ~  ,  ,
 =###,,,~~~~~~~~~~~~~~-------~~~~~~~~~~~~~~~,,###
08^Q,  ,  ~  ~  ~* ~  -  - I-  ~* ~  ~  ~  ~  ,
 =,,#######,,~~~~~~~~~-------~~~~~~~~~~~~~~~,,,,,
07  ,  ,  #  ,  ,* #  -  -  -  ~* ~  ~  ~  ~  ,
 =######,,,,,,,,####,,-------~~~~~~~~~~~~~~~###,,
06 4,4 #  ,  ,  ,  , *, *: *: *,  ~  ~  ~  ~  ,
 =,,,,,€,,,,,#########:::::::,,,,,~~~~~~~~~~,,€##
05 4, G,  ,  ,  ,  :  :  :  ,  ,  ~  ~  ~  ~  ,
 =,,,,,####,,,,,:::::::,,###€,,,~~~~~~~~~~~~,,,,,
04  ,  #  ,  #  :  :  ,  ,  #  ~  ~  ~  ~  ~  ,
 =,,#,,#,,####,,:::::::,,,,,#::----------::,,,€,,
03  #  ,  ,  #  ,  :  :  :  :  -  -  -  -  :  #
 =,,€,,,,,#,,#,,,,,,:::::::::::----------:::::#,,
02  ,  ,  #  ,  ,  ,  ,  ,  ,  ~  ~  ~  ~  :  :
 ====..%================================%.....%==
01  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
   A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P

Key: '=' High wall (8ft.), '~' Water, '#' Shrubs, '-' Wooden bridge, ':' Cobble Pathway, ',' Grass, '€' Tree, '%' Door, '.' Dirt, 'A-Z' Contestants

(G vs. Q, S vs. I) N X 

(A-8 will be in the mist will Polly move to F-8?)

Eldin doesn't see Gewonnt but, the behavior of the crowd lets you know he is right behind you.

Does The Tod get his Whelm back ? 6 no. He will multie slam Attack: 8 Damage: 15 and Attack: 17 Damage: 13

Grianne positions herself out of the mist but still next to polly if possible and bites at her prey for Attack: 16 Damage: 10

(What is Eider reacting to?)

Eldin hops down to B16 and thrusts his rapier into Gewonnt. "Not the exact conditions I had hoped for but this will be good enough."
To hit: 21 Dmg: 7 (I'm guessing Advantage and disadvantage cancelled out)

He then Dodges

(How does casting a spell that does no damage reveal my position? Mechanically according to the PHB pg. 177 The hiding status is only eliminated if you make a loud noise such as shouting a warning or breaking a vase. The spell components of Hunter's mark are verbal and I whispered it. Eider should not get a reaction. The ranger is an ambush class and not being able to cast hunters mark while hidden kind of goes against the grain of the flavor) 

Shmitty remains hidden and reaches into his rust colored Bag of Tricks and throws a 6 a rat (Goddammit) to L11. "Give him hell Superfly" Shmitty commands. The rat will move to Eider and Help Shmitty with his next attack. Billy Dee will do the same. 

I think I might have a way to solve this based on the hiding rule, if the DM can’t come to a solution, have Shmitty make a charisma or deception check vs a perception check. Just my two copper 

F8 is fine uncanny dodge on the slam the hit me

(Polly shouldn't be able to move. His speed was reduced to 0 till the start of his turn from Sentinel)