Off the top of my head you can randomly determine how many turns your units needs to stay on the table top before retreating off section 7,8,9, like 6 -12 turns.
Set up your units in section 4,5,6.
Start off with 3 PEFS in sections 1,2,3.
You can designate Activation rolls to generate on coming enemy reinforcements, besides a roll of 7. These enemy reinforcments would arrive in sections 1,2,3.
Play this scenario after you lose a Defend scenario.
Yep, that's a great way to do it. You can also have a section that is a key objective, like "Hold section 5 for 8 turns, and then retreat off the table from sections 7, 8 or 9 to give your comrades time to complete their retreat." You can also reduce your Investment Level if you want to simulate a retreat, and/or give the enemy Air Superiority. And if you want to max out the difficulty level make yourself Out of Supply and subject to Privation rules -- ouch!!
I wouldn't use PEFs, just have enemy forces come on the table and start moving towards your side until contact is made (ie: you are their PEFs). And give them fast moving reinforcements after towards the end so they don't spend their turns trying to catch up to you.
Diplomacy is the weapon of the civilized warrior. - A.T. Hun -- Robert Aspirin