Tenebris for me! I feel like Poison immunity would be too impactful on PvE, so maybe stun immunity instead.
Delugazar should have another. Perhaps a shield entrance to help it survive a bit 
Unmoveable for Soul Stealer and possibly for Oniblade but that might be a bit too much.
Shadowhunter and stormloch could use another but I can’t decide what.
Prismaryx, could give him hold ground
definitely stun counter.
rockoid: hold ground
With graviton wave as a move.
Shiva - Insomnia.
One of those anti-sleep
Tnb - Stun Immune.
We have Goo for poison.
Leogeist - Foggy Revenge /Fast-forward Revenge /Overwatch Revenge /Shocking Revenge.
Cloudy and Sting. Ultra Chrono Weakness.
Gloreo - Purify Entrance.
Its Holy-Bird.
Wraithcaptain - Insomnia.
Cant sleep the sleeper.
Dolphfreeze - Frozen Skin. -25% decelerate when attacked.
Solblaze - Shield Entrance.
He’s slow niche cloner.
Mechaviatan Harden Carapace /HG.
Nulltron - Stun Entrance.
Idk, something that stun.
Taloknight - Sleep Revenge.
How is an SE wolf stronger than him?
Sweetfeather - Sleep Revenge /Overwatch Entrance.
Just to make Exit Plan viable.
Soulstealer - Unmovable.
Delugazar - Shield Entrance / Union-HG.
Sobe - HG.
Malwing - Stun Immune.
give icefang - union excess force
Tenebris: Into darkness - poison and stun immunity when it has stealth (auto-poison stops the stealth on entrance so that still counters Tenebris).
Delugazar: Stunning entrance 
Nulltron: Stunning touch - stun enemies hit by 50s. Also change SS stun pulse → Instant headhunter.
Prismaryx: Battlefield formation - +50% attack when this monster has a shield.
Sweetfeather: Love potion - dies 200s after entering the battlefield and puts to sleep two random enemies.
Shivadragon : Defense mode/grit (perfect with hp sharing)
Nulltron : Swift stun pulse + Deus ex Machina (piercing+ unmovable)
Leogeist : Hold ground
Goldtail : 100 → 70 sec potent sleep + full defense stats + Holdground.
Malwing : sleep all revenge 
Shadowyrm : union (attack boost) + man eater
Kamiwyrm : Unmovable + Blizzard all
All SE bomber : unmovable or gravity field .
SE sleep all + Accel : fire → lava entrance, water → dreaming entrance.
Repulse -knockback SE : unwelcome entrance or unwelcome revenge.
That’d be perfect. It reminds me of Beritus saying “dragon food”.
Shiva-insomnia (anti 000)
Apollo-poison revenge ( fits link fire poison )
Tallo- haze revenge ( fits link shadow )
Shadowhunter-sleep entrance
Shiny shadowhunter- shield entrance
Stormlock-sleep revenge
Soulstealer-unmovable
Oniblade-unmovable
Leogeist- Hold Gound
Whraithcaptain- insomnia
Geomagnus-shield entrance
Solblaze-shield entrance ( too slow so it make sense )
Icefang- poisonous claws ( every atack will poison the enemy
Prismaryx- hold ground
Tenebris-stun immune 
poison immune would be perfect but i give up on it stun immune will do the job
Gloreonix-purify entrance ( sacred bird comes to rescue the oppresed 
Delugazar-threatening entrance ( slows enemys by 10% when entering the field