Dragons have a HELL a lot of HP. The reason that first one went down so fast is that (a) we had three players pounding on it so (b) when it was attacking one player the others could hit it freely and quickly and (c) it didn't focus on one player b/c it kept getting distracted and finally (d) I was using Sarpedon, my knight, with a 29 strength 25 agility, and had popped about 10 red pots so I was hitting very very quickly (I'd say about 2x/second) while the dragon attacked you two.
It can be done, they just have a mountain of HP, so you have to take about 5 tries at it before you finally kill it.
Do it quickly--their EXP reward is going way down as a part of tonight's update!