Apparently Horde Coop battle does count for it. Did it on horde capture all covers before killing Raam. It pop up for me. No luck doing it on normal Vs against lower or higher level players then me. I struggle to capture the last 2 small cover before finishing off the leader
Is better if both of you have Marcus of Old Man Marcus it helps with capturing covers faster. Thanks to horde boss having absurd amounts of hp your troops should capture it before killing it.