So we have a party of 5 and all at lvl 2 (rogue, sorcerer, barbarian, cavalier and cleric) and a random chart sent a CR 5 hydra at them which they took down way too fast. Granted it was just the one but I thought because the CR was so high I would only attack with one of the five heads. I see now this was a mistake, they went away happy and I felt like I did something wrong as GM. Any advice?