The secret Banshees egg is right at the beginning of the mission. Clear the first area at the start, making sure to hijack a Ghost, then look underneath the wooden platform over the water attached to the building on the shoreline. Underneath this platform is a wooden box. Hop in the Ghost and push the box to the side so that the wall behind its initial spawn is visible. Have a co-op partner stand here and get ready to press the switch that appears.
Now go into the very next area (where the Scorpion Tank is). Clear the enemies here. From where the floating platform is, go to the rocks on the waterline behind it.
Drop into the water and move to the left behind them (you will be out of bounds and have a 10-second death timer counting down). Look for a green switch at the base of the rocks by the water and hit it. After hitting the 1st switch a 2nd one will appear briefly on the wooden wall where the box was. Have the co-op partner hit this 2nd switch ASAP.
This will spawn four Banshees atop the wooden structure at the mission's start and pop the achievement for everyone in the game. Using them should help clear the mission without losing the Scorpion Tank on Legendary if you're struggling with Tank Beats Everything. However, per Classier in the comments: "spawning these banshees makes the AA guns invincible and the rest of the mission impossible to complete.What you have to do is destroy the AA guns before you spawn the banshees in order to use them to their full potential."