Delete "Fish in a Barrel." It is only challenging because it has no winning strategy.
Big horde of walkers? Don't rush in. Use ranged.
But most of them are armored. Use melee.
But don't use melee, there are too many close together.
Maybe you can retreat and use overwatch, and let them come to you a couple at a time? No, you start out backed into a corner, and the whole map is one small, empty rectangle.
Your only option is to micro-move as well as you can and hope the randomly generated spawns are favorable.