When king Henry also joins in (his army approacheth), you know it is time to cut your losses and get out before your enemies get too close with too much power. You retreat deeply back into Normandy, reinforce your army and get ready for yet another Franco-Angevin invasion. The combined army of the king and his vassals is the largest you have ever faced: it seems that any Norman intervention outside the duchy just pisses your king off and makes him want to slap you.
The invaders advance down the left bank of the river Dives, intent no doubt on cutting Normandy in half, and separating you from at least half of your realm. You can:
Appeal to your father-in-law to send troops from Flanders.
Send an appeal to the pope to intermediate.
Withdraw slowly before the enemy, in hopes that they will make a blunder you can take advantage of: they cannot bring you to battle unless you choose to.
Offer to discuss peace terms. Henry will agree, if you will come to a neutral place. (He knows that you will never allow yourself at this point to ever be placed within his power.)
Leave your army with competent commanders, and pay cousin Edward a visit in England, to rustle up more troops from there.