Walls have always been a bit of a hack in this game, and theres not much to do to prevent players from exploiting their use outside of mods. Glad you like the mission though! Always jazzed to hear people enjoying them

Probably my favorite mission out of all these EXOPS missions. The mission is pretty challenging, with tons of enemy attacks featuring engineers and recon bikes (something enemy doesn't use much in the base game), however if you make your base in the basin in the west of the map and wall off the southern entrance to the basin with sandbags, you can funnel all the enemy attacks through the basin's eastern entrance, making the mission much more manageable.