Trouble With All BB Party

I recently started a party with all BB's. I know all about how their unequipped attack is 2x their level and all that other stuff. I actually found it better to start using my BB's unequipped at level 7, even though their weapon provided an attack rating of 21 at that point. Anyway, I am currently level 8 and having a real difficult time with Astos. In general, the BB's do 1-2 damage per attack on him, with semi-frequent criticals of 25-30 damage. However, this is not enough to kill him. Is there something I am doing wrong or am I just not at a high enough level yet to kill him? What level should I get to before trying to defeat him? I haven't played a four party game in a while, but I do recall beating him by level 8 many times. The last game I played was a single fighter, so I really don't remember all this stuff. Thanks for the help.

Level up to 10 (which is when BBs get two extra attacks thus being able to deal significant damage to Astos) and keep reloading until his RUB attack fails.