To get hydra in free run mode of city trial you need too collect 3 parts this is how you get them:

PART 1 = break all dilapateded houses in one game(the iron blocks before the light house)
PART 2 = beat in total 1500 enemies of kirby melee.
PART 3 = beat more then 150 enemies in destruction derby(part 1 and 2 are in all the games of kirby melee and destruction derby)