When pitted against you, that is.

In any of the three fights against Bahamut (Isaaru, Belegmine, Yu Yevon), I noticed he just counts down to Mega Flare. The rest of them seem to just use their Aeon-special just like they would if they were on your side. But I've never seen Bahamut use Impulse ONCE against me.
And the funny thing is, it's the same in X-2!

Does anyone know if that ever happens?

As you could tell all I'm interested in is the mechanics...

All he does in X is use Mega Flare. However, in X-2, it's usually a string of his physical attacks and Impulse followed by a countdown to Mega Flare.