A Dragon Team you could use could be:
Old Hermit Jorge ----> TM Fill + Inc. ATK
Stag Knight ---------> AOE Def Down
Royal Guard ---------> AOE Nuke
Thesnail -------------> Heal + Inc. DEF
Frozen Banshee -----> Poison
I like Old Hermit Jorge over Scyl the Drakes because he can make your team move faster and increase the damage your team does.
The Dragon & The Waves Leading up to the Dragon are less forgiving in that particular Dungeon.
Scyl The Drakes is Great hero.
I just think you would be better off using the Old Hermit.
You can use Scyl in one of the other Dungeons.
In addition, You might want to level up a Warmaiden or Alexsander to replace Stag Knight.
Stag Knight is Spirit Affinity and Dragon 20 is Magic so he might do weak hits there.
Who ever you chose to be your Def Down hero.
My Recommended is to put them in Stun Gear.
Furthermore, Your Royal Guard build is not good.
You want to focus on getting 100% C.Rate + All The C.Damage you can muster.
I would recommend putting Thenasil in Shield set.
I would recommend putting Frozen Banshee in Lifesteal Set.
Lastly, The builds for your Ninja & Scyl are both wrong as well.
You need at least 200 ACC for Dungeon 20 Content.
Multiply the Stage your on times 10 to get the amount of ACC you need.
So if your planning to farm Dragon Stage 20.
It is 20 times 10 = 200 ACC.
Otherwise, your debuffs will most likely get resisted.