Mobile Garrison is a creature and does not have summoning sickness.
An opponent is unable to block and kill Mobile Garrison.
Steps
Declare Mobile Garrison as an attacker.
Mobile Garrison triggers, untapping Chromatic Orrery.
Move to your postcombat main phase.
Activate Chromatic Orrery by tapping it, adding at least ({5} magic symbol) .
Activate Aggravated Assault by paying ({5} magic symbol) , untapping all creatures you control and causing you to get an additional combat and main phase after this one.