Gears 5

I have been working through the horde solo and I find the easiest method is start on wave 50 and sign in multiple accounts and set them all to jack. Level up the demo class on your main as it can take down the boss. If you can take down the boss before your main dies otherwise spend the time hijacking enemies with jack then revive the demo and take down the boss. Best enemy combo I have found is swarmak along with either lambent or elite drones with lancers. Chainsaw doesn't aggro the enemies when hijacked so you can get up to 5 kills at a time then hijack same enemy on other Jack's till all are dead. Swarmak will die when all blisters are gone regardless of health so regen is not an issue. Be patient and you can get it done in 30mins with good fortune. Sometimes it takes a few starts to get the right boss. Works for 50% progress on this ach to date and all other similar operation achievements. Not sure about seriously as I am working through that.

I have the in game stats showing The Warren and Allfathers arena, both complete on master. I now have 2 mastered runs on horde frenzy. I am guessing it needs to be normal horde