Lowly armoured units will fatigue slower. Look for mercanaries with low armour rating. Since your in the marian reforms the light auxillia are lightly armoured and nimble. If they are short on cavalry you can harass them all day with the light auxillia if not you can have your cohorts hold the line in guard mode. Let the battle rage on until they are winded or tired. Then bring around your cavalry and roll up on them.

But your best off massing archers against Egyptians since they can wreak havoc on you with a missile heavy army. And wardogs are my #1 choice against chariots.