Mobile Garrison and Chromatic Orrery do not have summoning sickness.
An opponent cannot block and kill Mobile Garrison.
Steps
Declare Mobile Garrison and Najeela, if able, as attackers.
Mobile Garrison triggers, as well as Najeela if it was declared an attacker.
Resolve the Mobile Garrison trigger, untapping Chromatic Orrery.
Resolve the Najeela trigger, if applicable, creating a 1/1 Warrior creature token that is tapped and attacking.
Holding priority, activate Chromatic Orrery's first ability by tapping it, adding ({5} magic symbol) .
Activate Najeela by paying ({5} magic symbol) , untapping all attacking creatures, giving them trample, lifelink and haste, and gaining an additional combat phase after this one.