Mobile Garrison and Jegantha 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 Jegantha.
Resolve the Najeela trigger, if applicable, creating a 1/1 Warrior creature token that is tapped and attacking.
Holding priority, activate Jegantha by tapping it, adding ({W} magic symbol) ({U} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) that cannot be spent to pay generic mana costs.
Activate Najeela by paying ({W} magic symbol) ({U} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) , untapping all attacking creatures, giving them trample, lifelink and haste, and gaining an additional combat phase after this one.