You are always able to hit a Boomer one level above the one you can comfortably do if you team up with an alliance member. Team up/Rally at the same time and you both get the rewards from it.

Couple things to note:

• The auto joining mechanism gets the last squad of other players joining which 99% of the time will be their weakest one
• If you share the rally in alliance chat it instantly fills the rally as if it was waiting for the auto joiners
• Upgrade your Rally Square building to accommodate more players and do more damage (it goes on intervals of 5 levels: at level 10 you can have 6 total, at level 15 it is 7, at level 20 it is 8 and so on. The damage all players do is increased by 10% on these intervals)