×
Horse at 1, decoy at 2 facing NE

×What do you need, and for what?

×Entering from the South is one way you can ensure the bottommost plate is hit, although this isn't required.

×If I use the decoy to kill a brain, it'll be too far away to lure the horse and I don't see any way I can get the horse either to the water or to me without doing that.

If I use the decoy to press a plate near the horse, what usually happens is that the horse ends up in a position where I can't even get close enough to make it prioritize me over the decoy. The few times I've managed to avoid this, all I've been able to do is move the horse along the left side of the area.

If I use the decoy to kill the horse, leaving aside other issues, I don't have either a decoy or a horse to use to hit the plate past the force arrow.

If I use the decoy to hit the plate behind the arrow, then I have no decoy to help me kill the horse.

And entering from the south hasn't helped with hitting the last plate at all.

×So don't do any of those things.

×All four plates are pressed in order.