Id go for infernal king as you dont have to have traits unlocked (butbits desirable). A combo of hellcat/alchemist/infernal king can destroy any team out there because it keeps chaning.
Put 1 imervious troop in front of them to prevent getting frozen and you are good to go.
Suggested impervious troops: war spinx, manticore, behemoth.
As you dont have behemoth go for manti, its the safest.
Im using exactly that team, and im on the end game, i beat bone dragons etc without exceptions. Its a team for early to late game. Getting traits will only get that team strong but it isnt a must. Only must traits are for impervious troop in front.
Hellcat (No trait) Yellow/Purple (transform any to red for Hydra and Alchemist)
Alchemist*** Brown/Red (tranform any to yellow for Hellcat and Finley) and boost Hydra and Alchemist with Fire Link trait.
Hydra deals damage based on the X + the total life he has lost throughout the game x 2
So my stats for Hydra are 58 life and 36 damage.
If I cast him straight away, he deals 36 damage (0 boost)
If his life is down to 48, he deals 56 damage (10 x 2 boost)
However, if I then use mercy to give him say 20 life, even though his life says 68 the game views it as 68/78 (I.e. a give life spell not only increases the troops current life but also its maximum life) so he will still do 56 damage! I didn’t realise this for ages so I hope it is helpful.
As Nimhaim has said, if you heal the hydra then its boost is reset to zero.
So ideally you want to pair Hydra with a troop that can boost his life to keep him alive. Mercy, sunweaver, loyalty, justice, Lady Anariel. Emperina also isn’t a bad choice but as she heals as well as giving life, she resets your hydras boost every time she is used.
Hydra is risky because you want him to get hurt but not die. However, if you’re careful you can keep healing him and take out the entire team in one cast