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.