Cauldron of Souls and Hydra Interaction
Cauldron of Souls is not really a good card in a Hydra deck. 


  107.3f If a card in any zone other than the stack has an {X} in its mana cost, the value of {X} is treated as 0, even if the value of X is defined somewhere within its text.


That means that if the Hydra enters the battlefield in another way (e.g. from the graveyard through an animate effect, Flicker, or persist), it has no knowledge about the X you spend when you originally cast it (which may be turns ago, or have not even happened at all). Instead, X=0, the Hydra re-enters the battlefield with 0 +1/+1 counters and it instantly dies (unless it has a base toughness higher than 1 - it needs to survive the -1/-1 persist counter as well).

Otherwise, it would be a little overpowered, since +1/+1 and -1/-1 counters cancel each other out, and you would be able to repeatedly bring it back. (Though for 5 mana + the cost of the Hydra, you might expect such a powerful effect.)