I have contained all creatures in a room and casually walked up to the item/objective and casually walked out without losing any life with it. Squads B and F you say? Well I probably just teleport out of there.
While not at all going to argue with smart play sometimes winning an encounter - this particular "examp-othetical" does not hold to scrutiny well.
You are presuming a 9th level caster with two slots available at 5th at least **and** the mission objective separated from the adversaries their to challenge you to keep it.
So we must assume those adversaries have less capability to match their level than you? In the "objective challenge?"
honestly, at least one of my players if met with such a circumstance would stop and say "why don't they have competent protections here?" or at least wonder why the "adversaries" were making it this easy.
if later on the realized the macguffin recovery actually delivered a trojan curse to the "princess" or whatever sent us to seek the macguffin, they would go "oh yeah, now we know."