That Galato-Roman stack certainly is something to worry about. Based on my understanding of Narwhal's analysis of combat frontage, your best bet is to try to goad the enemy into attacking your most experienced phalanxes under your best commander in open terrain and fair weather. That, coupled with some luck, will mean that you'll have the best chance of avoiding a Thermopylae type situation where the Roman legions bear the brunt of the combat and rip your phalanxes apart.
I very much want you to win this. The Romans must taste blood and defeat.
I very much want you to win this. The Romans must taste blood and defeat.