Last night I sent my party (Average level 7) unprepared into a room with a juvenile black dragon (CR 7) and a pair of kobolds. It's a fairly spacious cavern with a number of stalagmites to use for cover and a deep grotto in the center. The dragon's tactics were basically fire breath weapon and then submerge for X-many rounds until he can spout acid again. The battle wound up having the dragon lose maybe thirty hp and the party was nearly wiped out when they decided to flee. They plan to come back later, but my question is is this setup justifiable for raising the EL of the encounter? A CR 7 creature shouldn't be so thoroughly mopping a group of 7th level adventurers, am I right?