We surely need to increase the mortality rate of immortals.
Therefore, we need to send all the ships available on the Expeditionary force, to the point the ships are filled. And yes, we need to have pikemen to counter immortal counterattacks at choice locations (high defense terrain at crossroads), supplemented with Ancient Cavalry to flank and ambush stray immortals coming for these entrenched units. This will be a heavy tactical operation, where every margin counts.
Again, this Group will target a very target rich environment, so we can just observe how the decoys work out, and how much they have left to spare.
Our prime afvantage, besides the qualitative and mobile, is one of surprise and projection of force. We decide turn, destination, unit count, follow ups, decoys, we decide on the pace of diplomacy, we decide on what they see and not see after the first strike and finally we have the strategic initiative.