BD deals 39-78 (average - 59) damage to phoenixes
Phoenixes deal 62-90 (average 76) damage to BD
Considering that the initiative of phoenixes is 12 versus 7 for the BD, the phoenixes get the first strike.
BD | P
400 |275\275
324 |275\216
248 |275\157
172 |275\98
96 |275\39
20 |225
0 |166
This is without considering luck (both units have 0) and morale. And the BD loses morale time and again!
So the probability that 1 BD will take out 2 phoenixes is lower than the probability that the phoenixes will take him out without losses.
But of course, I am not claiming that phoenixes are more effective on the field than the BD; I'll say it again - everything depends on the map.