What kind of team are you trying to build exactly? You've got cardiel, who's indicative of a meatgrinder team and serris and deacon are both strong cards in a speed nuke setup, but you're not running the offense that either of those require, nor the healing and defense you would need for a survival build.

Speed  Nuking  Set  up:  It  would  be  Classical  Arena  team

Leader:  Deacon  -  Speed  Aura  19%

1st  move:  Apothecary  --------->  TM  Fill  +  Inc.  SPD

2nd  move:  Old  Hermit  Jorg  -->  TM  Fill  +  Inc.  ATK

3rd  move:  Deacon  ------------>  TM  Fill  +  TM  Down  +  AOE  DEF  Down

4th  move:  Lua  ---------------->  AOE  Nuke

Alternative  Speed  Nuking  Set  up:  Same  as  previous team  with  few  changes.

Leader:  High  Khatun  -  Speed  Aura  19%

Swap  out  Apothecary for  High  Khatun

Swap  out  Deacon  for  Madam  Serris

Stall  Set  up:  It  would  be  a  Rally  Revive  team.

Leader:  Cardiel  -  Speed  Aura  19%

1st  move:  Old  Hermit  Jorg  --->  TM  +  Inc.  ATK

2nd  move:  Cardiel  ------------>  Inc.  C.RATE  +  Inc.  C.DMG +  Rally

3rd  move:  Sycl  of  Drakes  ---->  AOE  Stun  +  Passive  Heal

4th  move:  Melga  ------------->  Team  Shield  +  C.Heals

Work-In-Progress - Blender Set  up:  You  are  close  to  making  one,  but  lack  hero.

Leader:  Cardiel  -  Speed  Aura  19%

1st  move:  Old  Hermit  Jorg  --->  TM  +  Inc.  ATK

2nd  move:  Cardiel  ------------>  Inc.  C.RATE  +  Inc.  C.DMG +  Rally

3rd  move:  Skartorsis  --------->  A1  AOE  Nuke

4th  move:  ?????   ------------->  A1  AOE  Nuke

Hello OP

''Madame Serris, Cardiel, and Deacon Armstrong for my team but really am not sure if I should keep Apothecary as my fourth member or switch him. ''

madam serris + deacon? why? both champions provide a unique thing each, but they have one thing in common, aoe defence down, having them both in the same team isn't necessarily optimal unless you have a solid core trio 

as for adding apo in this comp, who is the dps champion?

''I have Melga SteelGirdle, Luria, Lua, Old Hermit Jorrg, RearGuard Sergeant, Scyl, Skartorsis, Tayrel, Thenasil, and High Khatun as possible alternates. ''

out of these stated champs lua should be your aoe dps champion, deacon your speed lead and aoe def down, apo you booster, and cardiel can be fit in there as the 4th champ, or in some cases scyl

good luck!

my 2 cents:

Why bother with this analysis paralysis?  We are in BOTLAND!  Go with the flow and hit the EZ ones!